feat(bootloader): add the possibility to specify extra components directories

examples/custom_bootloader/bootloader_extra_dir/CMakeLists.txt

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+# For more information about build system see
+# https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/build-system.html
+# The following five lines of boilerplate have to be in your project's
+# CMakeLists in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.16)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+
+set(BOOTLOADER_EXTRA_COMPONENT_DIRS "${CMAKE_CURRENT_LIST_DIR}/extra_bootloader_components/")
+
+project(main)

examples/custom_bootloader/bootloader_extra_dir/README.md

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+| Supported Targets | ESP32 | ESP32-C2 | ESP32-C3 | ESP32-C5 | ESP32-C6 | ESP32-C61 | ESP32-H2 | ESP32-P4 | ESP32-S2 | ESP32-S3 |
+| ----------------- | ----- | -------- | -------- | -------- | -------- | --------- | -------- | -------- | -------- | -------- |
+
+# Bootloader extra component
+
+(See the README.md file in the upper level for more information about bootloader examples.)
+
+The purpose of this example is to show how to add a custom directory that contains a component to the bootloader build.
+
+Registering extra components for the bootloader can be done thanks to the CMake variable `BOOTLOADER_EXTRA_COMPONENT_DIRS`. It works the same way as the application's `EXTRA_COMPONENT_DIRS`, it can either refer to a directory that contains several components, either refer to a single component.
+
+## Usage of this example:
+
+Simply compile it:
+```
+idf.py build
+```
+
+Then flash it and open the monitor with the following command:
+```
+idf.py flash monitor
+```
+
+If everything went well, the bootloader should output the following message:
+```
+I (60) EXTRA: This function is called from an extra component
+```
+
+And finally the application will start and show the message:
+```
+User application is loaded and running.
+```
+
+## Organization of this example
+
+This project contains an application, in the `main` directory that represents an application. It also contains a `bootloader_components` that contains a component compiled and linked with the bootloader. This `bootloader_components` can contain several components, each of them would be in a different directory.
+
+The directory `extra_bootloader_components/extra_component/` contains a component that is meant to be included in the bootloader build. To do so, the variable `BOOTLOADER_EXTRA_COMPONENT_DIRS` is set from the `CMakeLists.txt` file.
+
+Below is a short explanation of files in the project folder.
+
+```
+├── CMakeLists.txt             Defines the `BOOTLOADER_EXTRA_COMPONENT_DIRS` variable
+├── main
+│   ├── CMakeLists.txt
+│   └── main.c                 User application
+├── bootloader_components
+│   └── my_boot_hooks
+│       ├── CMakeLists.txt
+│       └── hooks.c            Implementation of the hooks to execute on boot
+├── extra_bootloader_components
+│   └── extra_component
+│       ├── CMakeLists.txt
+│       └── extra_component.c  Implementation of the extra component
+└── README.md                  This is the file you are currently reading
+```

examples/custom_bootloader/bootloader_extra_dir/bootloader_components/my_boot_hooks/CMakeLists.txt

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+idf_component_register(SRCS "hooks.c"
+                       REQUIRES extra_component)
+
+# We need to force GCC to integrate this static library into the
+# bootloader link. Indeed, by default, as the hooks in the bootloader are weak,
+# the linker would just ignore the symbols in the extra. (i.e. not strictly
+# required)
+# To do so, we need to define the symbol (function) `bootloader_hooks_include`
+# within hooks.c source file.

examples/custom_bootloader/bootloader_extra_dir/bootloader_components/my_boot_hooks/hooks.c

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+/*
+ * SPDX-FileCopyrightText: 2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+extern void bootloader_extra_dir_function(void);
+
+/* Function used to tell the linker to include this file
+ * with all its symbols.
+ */
+void bootloader_hooks_include(void){
+}
+
+void bootloader_after_init(void) {
+    bootloader_extra_dir_function();
+}

examples/custom_bootloader/bootloader_extra_dir/extra_bootloader_components/extra_component/CMakeLists.txt

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+idf_component_register(SRCS "extra_component.c")

examples/custom_bootloader/bootloader_extra_dir/extra_bootloader_components/extra_component/extra_component.c

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+/*
+ * SPDX-FileCopyrightText: 2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include "esp_log.h"
+
+void bootloader_extra_dir_function(void)
+{
+    ESP_LOGI("EXTRA", "This function is called from an extra component");
+}

examples/custom_bootloader/bootloader_extra_dir/main/CMakeLists.txt

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+idf_component_register(SRCS "bootloader_hooks_example_main.c"
+                    INCLUDE_DIRS ".")

examples/custom_bootloader/bootloader_extra_dir/main/bootloader_hooks_example_main.c

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+/*
+ * SPDX-FileCopyrightText: 2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include <stdio.h>
+
+void app_main(void)
+{
+    printf("User application is loaded and running.\n");
+}

examples/custom_bootloader/bootloader_extra_dir/pytest_bootloader_extra_dir.py

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+# SPDX-FileCopyrightText: 2024 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: CC0-1.0
+import pytest
+from pytest_embedded import Dut
+
+
+@pytest.mark.supported_targets
+@pytest.mark.generic
+def test_custom_bootloader_extra_component(dut: Dut) -> None:
+    dut.expect_exact('This function is called from an extra component')