This document explains the Espressif IoT Development Framework build system and the concept of “components”
Read this document if you want to know how to organise a new ESP-IDF project.
We recommend using the esp-idf-template project as a starting point for your project.
Using the Build System¶
The esp-idf README file contains a description of how to use the build system to build your project.
An ESP-IDF project can be seen as an amalgamation of a number of components. For example, for a webserver that shows the current humidity, there could be:
- The ESP32 base libraries (libc, rom bindings etc)
- The WiFi drivers
- A TCP/IP stack
- The FreeRTOS operating system
- A webserver
- A driver for the humidity sensor
- Main code tying it all together
ESP-IDF makes these components explicit and configurable. To do that, when a project is compiled, the build environment will look up all the components in the ESP-IDF directories, the project directories and (optionally) in additional custom component directories. It then allows the user to configure the ESP-IDF project using a a text-based menu system to customize each component. After the components in the project are configured, the build process will compile the project.
- A “project” is a directory that contains all the files and configuration to build a single “app” (executable), as well as additional supporting output such as a partition table, data/filesystem partitions, and a bootloader.
- “Project configuration” is held in a single file called sdkconfig in the root directory of the project. This configuration file is modified via
make menuconfigto customise the configuration of the project. A single project contains exactly one project configuration.
- An “app” is an executable which is built by esp-idf. A single project will usually build two apps - a “project app” (the main executable, ie your custom firmware) and a “bootloader app” (the initial bootloader program which launches the project app).
- “components” are modular pieces of standalone code which are compiled into static libraries (.a files) and linked into an app. Some are provided by esp-idf itself, others may be sourced from other places.
Some things are not part of the project:
- “ESP-IDF” is not part of the project. Instead it is standalone, and linked to the project via the
IDF_PATHenvironment variable which holds the path of the
esp-idfdirectory. This allows the IDF framework to be decoupled from your project.
- The toolchain for compilation is not part of the project. The toolchain should be installed in the system command line PATH, or the path to the toolchain can be set as part of the compiler prefix in the project configuration.
An example project directory tree might look like this:
- myProject/ - Makefile - sdkconfig - components/ - component1/ - component.mk - Kconfig - src1.c - component2/ - component.mk - Kconfig - src1.c - include/ - component2.h - main/ - src1.c - src2.c - component.mk - build/
This example “myProject” contains the following elements:
- A top-level project Makefile. This Makefile set the
PROJECT_NAMEvariable and (optionally) defines other project-wide make variables. It includes the core
$(IDF_PATH)/make/project.mkmakefile which implements the rest of the ESP-IDF build system.
- “sdkconfig” project configuration file. This file is created/updated when “make menuconfig” runs, and holds configuration for all of the components in the project (including esp-idf itself). The “sdkconfig” file may or may not be added to the source control system of the project.
- Optional “components” directory contains components that are part of the project. A project does not have to contain custom components of this kind, but it can be useful for structuring reusable code or including third party components that aren’t part of ESP-IDF.
- “main” directory is a special “pseudo-component” that contains source code for the project itself. “main” is a default name, the Makefile variable
COMPONENT_DIRSincludes this component but you can modify this variable (or set
EXTRA_COMPONENT_DIRS) to look for components in other places.
- “build” directory is where build output is created. After the make process is run, this directory will contain interim object files and libraries as well as final binary output files. This directory is usually not added to source control or distributed with the project source code.
Component directories contain a component makefile -
component.mk. This may contain variable definitions
to control the build process of the component, and its integration into the overall project. See Component Makefiles for more details.
Each component may also include a
Kconfig file defining the component configuration options that can be set via the project configuration. Some components may also include
Makefile.projbuild files, which are special files for overriding parts of the project.
Each project has a single Makefile that contains build settings for the entire project. By default, the project Makefile can be quite minimal.
Minimal Example Makefile¶
PROJECT_NAME := myProject include $(IDF_PATH)/make/project.mk
Mandatory Project Variables¶
PROJECT_NAME: Name of the project. Binary output files will use this name - ie myProject.bin, myProject.elf.
Optional Project Variables¶
These variables all have default values that can be overridden for custom behaviour. Look in
make/project.mk for all of the implementation details.
PROJECT_PATH: Top-level project directory. Defaults to the directory containing the Makefile. Many other project variables are based on this variable. The project path cannot contain spaces.
BUILD_DIR_BASE: The build directory for all objects/libraries/binaries. Defaults to
COMPONENT_DIRS: Directories to search for components. Defaults to $(IDF_PATH)/components, $(PROJECT_PATH)/components,
EXTRA_COMPONENT_DIRS. Override this variable if you don’t want to search for components in these places.
EXTRA_COMPONENT_DIRS: Optional list of additional directories to search for components.
COMPONENTS: A list of component names to build into the project. Defaults to all components found in the COMPONENT_DIRS directories.
EXCLUDE_COMPONENTS: Optional list of component names to exclude during the build process. Note that this decreases build time, but not binary size.
TEST_EXCLUDE_COMPONENTS: Optional list of component names to exclude during the build process of unit tests.
Any paths in these Makefile variables should be absolute paths. You can convert relative paths using
$(IDF_PATH)/xxx, or use the Make function
These variables should all be set before the line
include $(IDF_PATH)/make/project.mk in the Makefile.
Each project contains one or more components, which can either be part of esp-idf or added from other component directories.
A component is any directory that contains a
Searching for Components¶
The list of directories in
COMPONENT_DIRS is searched for the project’s components. Directories in this list can either be components themselves (ie they contain a component.mk file), or they can be top-level directories whose subdirectories are components.
make list-components target dumps many of these variables and can help debug the discovery of component directories.
Multiple components with the same name¶
When esp-idf is collecting all the components to compile, it will do this in the order specified by
COMPONENT_DIRS; by default, this means the
idf components first, the project components second and optionally the components in
EXTRA_COMPONENT_DIRS last. If two or more of these directories
contain component subdirectories with the same name, the component in the last place searched is used. This allows, for example, overriding esp-idf components
with a modified version by simply copying the component from the esp-idf component directory to the project component tree and then modifying it there.
If used in this way, the esp-idf directory itself can remain untouched.
Minimal Component Makefile¶
component.mk file is an empty file(!). If the file is empty, the default component behaviour is set:
- All source files in the same directory as the makefile (
*.S) will be compiled into the component library
- A sub-directory “include” will be added to the global include search path for all other components.
- The component library will be linked into the project app.
See example component makefiles for more complete component makefile examples.
Note that there is a difference between an empty
component.mk file (which invokes default component build behaviour) and no
component.mk file (which means no default component build behaviour will occur.) It is possible for a component to have no component.mk file, if it only contains other files which influence the project configuration or build process.
Preset Component Variables¶
The following component-specific variables are available for use inside
component.mk, but should not be modified:
COMPONENT_PATH: The component directory. Evaluates to the absolute path of the directory containing
component.mk. The component path cannot contain spaces.
COMPONENT_NAME: Name of the component. Defaults to the name of the component directory.
COMPONENT_BUILD_DIR: The component build directory. Evaluates to the absolute path of a directory inside $(BUILD_DIR_BASE) where this component’s source files are to be built. This is also the Current Working Directory any time the component is being built, so relative paths in make targets, etc. will be relative to this directory.
COMPONENT_LIBRARY: Name of the static library file (relative to the component build directory) that will be built for this component. Defaults to
The following variables are set at the project level, but exported for use in the component build:
PROJECT_NAME: Name of the project, as set in project Makefile
PROJECT_PATH: Absolute path of the project directory containing the project Makefile.
COMPONENTS: Name of all components that are included in this build.
CONFIG_*: Each value in the project configuration has a corresponding variable available in make. All names begin with
OBJCOPY: Full paths to each tool from the gcc xtensa cross-toolchain.
HOSTAR: Full names of each tool from the host native toolchain.
IDF_VER: ESP-IDF version, retrieved from either
$(IDF_PATH)/version.txtfile (if present) else using git command
git describe. Recommended format here is single liner that specifies major IDF release version, e.g.
v2.0for a tagged release or
v2.0-275-g0efaa4ffor an arbitrary commit. Application can make use of this by calling
If you modify any of these variables inside
component.mk then this will not prevent other components from building but it may make your component hard to build and/or debug.
Optional Project-Wide Component Variables¶
The following variables can be set inside
component.mk to control build settings across the entire project:
COMPONENT_ADD_INCLUDEDIRS: Paths, relative to the component directory, which will be added to the include search path for all components in the project. Defaults to
includeif not overridden. If an include directory is only needed to compile this specific component, add it to
COMPONENT_ADD_LDFLAGS: Add linker arguments to the LDFLAGS for the app executable. Defaults to
-l$(COMPONENT_NAME). If adding pre-compiled libraries to this directory, add them as absolute paths - ie $(COMPONENT_PATH)/libwhatever.a
COMPONENT_DEPENDS: Optional list of component names that should be compiled before this component. This is not necessary for link-time dependencies, because all component include directories are available at all times. It is necessary if one component generates an include file which you then want to include in another component. Most components do not need to set this variable.
COMPONENT_ADD_LINKER_DEPS: Optional list of component-relative paths to files which should trigger a re-link of the ELF file if they change. Typically used for linker script files and binary libraries. Most components do not need to set this variable.
The following variable only works for components that are part of esp-idf itself:
COMPONENT_SUBMODULES: Optional list of git submodule paths (relative to COMPONENT_PATH) used by the component. These will be checked (and initialised if necessary) by the build process. This variable is ignored if the component is outside the IDF_PATH directory.
Optional Component-Specific Variables¶
The following variables can be set inside
component.mk to control the build of that component:
COMPONENT_PRIV_INCLUDEDIRS: Directory paths, must be relative to the component directory, which will be added to the include search path for this component’s source files only.
COMPONENT_EXTRA_INCLUDES: Any extra include paths used when compiling the component’s source files. These will be prefixed with ‘-I’ and passed as-is to the compiler. Similar to the
COMPONENT_PRIV_INCLUDEDIRSvariable, except these paths are not expanded relative to the component directory.
COMPONENT_SRCDIRS: Directory paths, must be relative to the component directory, which will be searched for source files (
*.S). Defaults to ‘.’, ie the component directory itself. Override this to specify a different list of directories which contain source files.
COMPONENT_OBJS: Object files to compile. Default value is a .o file for each source file that is found in
COMPONENT_SRCDIRS. Overriding this list allows you to exclude source files in
COMPONENT_SRCDIRSthat would otherwise be compiled. See Specifying source files
COMPONENT_EXTRA_CLEAN: Paths, relative to the component build directory, of any files that are generated using custom make rules in the component.mk file and which need to be removed as part of
make clean. See Source Code Generation for an example.
COMPONENT_OWNCLEANTARGET: These targets allow you to fully override the default build behaviour for the component. See Fully Overriding The Component Makefile for more details.
COMPONENT_CONFIG_ONLY: If set, this flag indicates that the component produces no built output at all (ie
COMPONENT_LIBRARYis not built), and most other component variables are ignored. This flag is used for IDF internal components which contain only
Makefile.projbuildfiles to configure the project, but no source files.
CFLAGS: Flags passed to the C compiler. A default set of
CFLAGSis defined based on project settings. Component-specific additions can be made via
CFLAGS +=. It is also possible (although not recommended) to override this variable completely for a component.
CPPFLAGS: Flags passed to the C preprocessor (used for .c, .cpp and .S files). A default set of
CPPFLAGSis defined based on project settings. Component-specific additions can be made via
CPPFLAGS +=. It is also possible (although not recommended) to override this variable completely for a component.
CXXFLAGS: Flags passed to the C++ compiler. A default set of
CXXFLAGSis defined based on project settings. Component-specific additions can be made via
CXXFLAGS +=. It is also possible (although not recommended) to override this variable completely for a component.
To apply compilation flags to a single source file, you can add a variable override as a target, ie:
apps/dhcpserver.o: CFLAGS += -Wno-unused-variable
This can be useful if there is upstream code that emits warnings.
Each component can also have a Kconfig file, alongside
component.mk. This contains contains
configuration settings to add to the “make menuconfig” for this component.
These settings are found under the “Component Settings” menu when menuconfig is run.
To create a component KConfig file, it is easiest to start with one of the KConfig files distributed with esp-idf.
For an example, see Adding conditional configuration.
ESP-IDF build systems adds the following C preprocessor definitions on the command line:
ESP_PLATFORM— Can be used to detect that build happens within ESP-IDF.
IDF_VER— ESP-IDF version, see Preset Component Variables for more details.
Build Process Internals¶
Top Level: Project Makefile¶
- “make” is always run from the project directory and the project makefile, typically named Makefile.
- The project makefile sets
PROJECT_NAMEand optionally customises other optional project variables
- The project makefile includes
$(IDF_PATH)/make/project.mkwhich contains the project-level Make logic.
project.mkfills in default project-level make variables and includes make variables from the project configuration. If the generated makefile containing project configuration is out of date, then it is regenerated (via targets in
project_config.mk) and then the make process restarts from the top.
project.mkbuilds a list of components to build, based on the default component directories or a custom list of components set in optional project variables.
- Each component can set some optional project-wide component variables. These are included via generated makefiles named
component_project_vars.mk- there is one per component. These generated makefiles are included into
project.mk. If any are missing or out of date, they are regenerated (via a recursive make call to the component makefile) and then the make process restarts from the top.
- Makefile.projbuild files from components are included into the make process, to add extra targets or configuration.
- By default, the project makefile also generates top-level build & clean targets for each component and sets up app and clean targets to invoke all of these sub-targets.
- In order to compile each component, a recursive make is performed for the component makefile.
To better understand the project make process, have a read through the
project.mk file itself.
Second Level: Component Makefiles¶
- Each call to a component makefile goes via the
- This component wrapper includes all component
Makefile.componentbuildfiles, making any recipes, variables etc in these files available to every component.
component_wrapper.mkis called with the current directory set to the component build directory, and the
COMPONENT_MAKEFILEvariable is set to the absolute path to
component_wrapper.mksets default values for all component variables, then includes the component.mk file which can override or modify these.
COMPONENT_OWNCLEANTARGETare not defined, default build and clean targets are created for the component’s source files and the prerequisite
COMPONENT_LIBRARYstatic library file.
component_project_vars.mkfile has its own target in
component_wrapper.mk, which is evaluated from
project.mkif this file needs to be rebuilt due to changes in the component makefile or the project configuration.
To better understand the component make process, have a read through the
component_wrapper.mk file and some of the
component.mk files included with esp-idf.
Running Make Non-Interactively¶
make in a situation where you don’t want interactive prompts (for example: inside an IDE or an automated build system) append
BATCH_BUILD=1 to the make arguments (or set it as an environment variable).
BATCH_BUILD implies the following:
- Verbose output (same as
V=1, see below). If you don’t want verbose output, also set
- If the project configuration is missing new configuration items (from new components or esp-idf updates) then the project use the default values, instead of prompting the user for each item.
- If the build system needs to invoke
menuconfig, an error is printed and the build fails.
Debugging The Make Process¶
Some tips for debugging the esp-idf build system:
V=1to the make arguments (or setting it as an environment variable) will cause make to echo all commands executed, and also each directory as it is entered for a sub-make.
make -wwill cause make to echo each directory as it is entered for a sub-make - same as
V=1but without also echoing all commands.
make --trace(possibly in addition to one of the above arguments) will print out every target as it is built, and the dependency which caused it to be built.
make -pprints a (very verbose) summary of every generated target in each makefile.
For more debugging tips and general make information, see the GNU Make Manual.
Warning On Undefined Variables¶
By default, the build process will print a warning if an undefined variable is referenced (like
$(DOES_NOT_EXIST)). This can be useful to find errors in variable names.
If you don’t want this behaviour, it can be disabled in menuconfig’s top level menu under SDK tool configuration.
Note that this option doesn’t trigger a warning if
ifndef are used in Makefiles.
Overriding Parts of the Project¶
For components that have build requirements that must be evaluated in the top-level
project make pass, you can create a file called
Makefile.projbuild in the
component directory. This makefile is included when
project.mk is evaluated.
For example, if your component needs to add to CFLAGS for the entire
project (not just for its own source files) then you can set
CFLAGS += in Makefile.projbuild.
Makefile.projbuild files are used heavily inside esp-idf, for defining project-wide build features such as
esptool.py command line arguments and the
bootloader “special app”.
Makefile.projbuild isn’t necessary for the most common component uses - such as adding include directories to the project, or LDFLAGS to the final linking step. These values can be customised via the
component.mk file itself. See Optional Project-Wide Component Variables for details.
Take care when setting variables or targets in this file. As the values are included into the top-level project makefile pass, they can influence or break functionality across all components!
This is an equivalent to
Makefile.projbuild for component configuration KConfig files. If you want to include
configuration options at the top-level of menuconfig, rather than inside the “Component Configuration” sub-menu, then these can be defined in the KConfig.projbuild file alongside the
Take care when adding configuration values in this file, as they will be included across the entire project configuration. Where possible, it’s generally better to create a KConfig file for component configuration.
For components that e.g. include tools to generate source files from other files, it is necessary to be able to add recipes, macros or variable definitions
into the component build process of every components. This is done by having a
Makefile.componentbuild in a component directory. This file gets included
component_wrapper.mk, before the
component.mk of the component is included. As with the Makefile.projbuild, take care with these files: as they’re
included in each component build, a
Makefile.componentbuild error may only show up when compiling an entirely different component.
Some special components which contain no source files, only
Makefile.projbuild, may set the flag
COMPONENT_CONFIG_ONLY in the component.mk file. If this flag is set, most other component variables are ignored and no build step is run for the component.
Example Component Makefiles¶
Because the build environment tries to set reasonable defaults that will work most of the time, component.mk can be very small or even empty (see Minimal Component Makefile). However, overriding component variables is usually required for some functionality.
Here are some more advanced examples of
Adding source directories¶
By default, sub-directories are ignored. If your project has sources in sub-directories
instead of in the root of the component then you can tell that to the build
system by setting
COMPONENT_SRCDIRS := src1 src2
This will compile all source files in the src1/ and src2/ sub-directories instead.
Specifying source files¶
The standard component.mk logic adds all .S and .c files in the source
directories as sources to be compiled unconditionally. It is possible
to circumvent that logic and hard-code the objects to be compiled by
manually setting the
COMPONENT_OBJS variable to the name of the
objects that need to be generated:
COMPONENT_OBJS := file1.o file2.o thing/filea.o thing/fileb.o anotherthing/main.o COMPONENT_SRCDIRS := . thing anotherthing
COMPONENT_SRCDIRS must be set as well.
Adding conditional configuration¶
The configuration system can be used to conditionally compile some files
depending on the options selected in
make menuconfig. For this, ESP-IDF
has the compile_only_if and compile_only_if_not macros:
config FOO_ENABLE_BAR bool "Enable the BAR feature." help This enables the BAR feature of the FOO component.
As can be seen in the example, the
compile_only_if macro takes a condition and a
list of object files as parameters. If the condition is true (in this case: if the
BAR feature is enabled in menuconfig) the object files (in this case: bar.o) will
always be compiled. The opposite goes as well: if the condition is not true, bar.o
will never be compiled.
compile_only_if_not does the opposite: compile if the
condition is false, not compile if the condition is true.
This can also be used to select or stub out an implementation, as such:
config ENABLE_LCD_OUTPUT bool "Enable LCD output." help Select this if your board has a LCD. config ENABLE_LCD_CONSOLE bool "Output console text to LCD" depends on ENABLE_LCD_OUTPUT help Select this to output debugging output to the lcd config ENABLE_LCD_PLOT bool "Output temperature plots to LCD" depends on ENABLE_LCD_OUTPUT help Select this to output temperature plots
# If LCD is enabled, compile interface to it, otherwise compile dummy interface $(call compile_only_if,$(CONFIG_ENABLE_LCD_OUTPUT),lcd-real.o lcd-spi.o) $(call compile_only_if_not,$(CONFIG_ENABLE_LCD_OUTPUT),lcd-dummy.o) #We need font if either console or plot is enabled $(call compile_only_if,$(or $(CONFIG_ENABLE_LCD_CONSOLE),$(CONFIG_ENABLE_LCD_PLOT)), font.o)
Note the use of the Make ‘or’ function to include the font file. Other substitution functions, like ‘and’ and ‘if’ will also work here. Variables that do not come from menuconfig can also be used: ESP-IDF uses the default Make policy of judging a variable which is empty or contains only whitespace to be false while a variable with any non-whitespace in it is true.
(Note: Older versions of this document advised conditionally adding object file names to
COMPONENT_OBJS. While this still is possible, this will only work when all object
files for a component are named explicitely, and will not clean up deselected object files
make clean pass.)
Source Code Generation¶
Some components will have a situation where a source file isn’t supplied with the component itself but has to be generated from another file. Say our component has a header file that consists of the converted binary data of a BMP file, converted using a hypothetical tool called bmp2h. The header file is then included in as C source file called graphics_lib.c:
COMPONENT_EXTRA_CLEAN := logo.h graphics_lib.o: logo.h logo.h: $(COMPONENT_PATH)/logo.bmp bmp2h -i $^ -o $@
In this example, graphics_lib.o and logo.h will be generated in the current directory (the build directory) while logo.bmp comes with the component and resides under the component path. Because logo.h is a generated file, it needs to be cleaned when make clean is called which why it is added to the COMPONENT_EXTRA_CLEAN variable.
Because logo.h is a generated file, it needs to be cleaned when make clean is called which why it is added to the COMPONENT_EXTRA_CLEAN variable.
Adding logo.h to the
graphics_lib.o dependencies causes it to be
graphics_lib.c is compiled.
If a a source file in another component included
logo.h, then this
component’s name would have to be added to the other component’s
COMPONENT_DEPENDS list to ensure that the components were built
Embedding Binary Data¶
Sometimes you have a file with some binary or text data that you’d like to make available to your component - but you don’t want to reformat the file as C source.
You can set a variable COMPONENT_EMBED_FILES in component.mk, giving the names of the files to embed in this way:
COMPONENT_EMBED_FILES := server_root_cert.der
Or if the file is a string, you can use the variable COMPONENT_EMBED_TXTFILES. This will embed the contents of the text file as a null-terminated string:
COMPONENT_EMBED_TXTFILES := server_root_cert.pem
The file’s contents will be added to the .rodata section in flash, and are available via symbol names as follows:
extern const uint8_t server_root_cert_pem_start asm("_binary_server_root_cert_pem_start"); extern const uint8_t server_root_cert_pem_end asm("_binary_server_root_cert_pem_end");
The names are generated from the full name of the file, as given in COMPONENT_EMBED_FILES. Characters /, ., etc. are replaced with underscores. The _binary prefix in the symbol name is added by objcopy and is the same for both text and binary files.
For an example of using this technique, see protocols/https_request - the certificate file contents are loaded from the text .pem file at compile time.
Fully Overriding The Component Makefile¶
Obviously, there are cases where all these recipes are insufficient for a
certain component, for example when the component is basically a wrapper
around another third-party component not originally intended to be
compiled under this build system. In that case, it’s possible to forego
the esp-idf build system entirely by setting COMPONENT_OWNBUILDTARGET and
possibly COMPONENT_OWNCLEANTARGET and defining your own targets named
target. The build target can do anything as long as it creates
$(COMPONENT_LIBRARY) for the project make process to link into the app binary.
(Actually, even this is not strictly necessary - if the COMPONENT_ADD_LDFLAGS variable is overridden then the component can instruct the linker to link other binaries instead.)
Custom sdkconfig defaults¶
For example projects or other projects where you don’t want to specify a full sdkconfig configuration, but you do want to override some key values from the esp-idf defaults, it is possible to create a file
sdkconfig.defaults in the project directory. This file will be used when running
make defconfig, or creating a new config from scratch.
To override the name of this file, set the
SDKCONFIG_DEFAULTS environment variable.
Save flash arguments¶
There’re some scenarios that we want to flash the target board without IDF. For this case we want to save the built binaries, esptool.py and esptool write_flash arguments. It’s simple to write a script to save binaries and esptool.py. We can use command
make print_flash_cmd, it will print the flash arguments:
--flash_mode dio --flash_freq 40m --flash_size detect 0x1000 bootloader/bootloader.bin 0x10000 example_app.bin 0x8000 partition_table_unit_test_app.bin
Then use flash arguments as the arguemnts for esptool write_flash arguments:
python esptool.py --chip esp32 --port /dev/ttyUSB0 --baud 921600 --before default_reset --after hard_reset write_flash -z --flash_mode dio --flash_freq 40m --flash_size detect 0x1000 bootloader/bootloader.bin 0x10000 example_app.bin 0x8000 partition_table_unit_test_app.bin
Building the Bootloader¶
The bootloader is built by default as part of “make all”, or can be built standalone via “make bootloader-clean”. There is also “make bootloader-list-components” to see the components included in the bootloader build.
The component in IDF components/bootloader is special, as the second stage bootloader is a separate .ELF and .BIN file to the main project. However it shares its configuration and build directory with the main project.
This is accomplished by adding a subproject under components/bootloader/subproject. This subproject has its own Makefile, but it expects to be called from the project’s own Makefile via some glue in the components/bootloader/Makefile.projectbuild file. See these files for more details.