bootloader: override the 2nd stage bootloader

Add the possibility to have user bootloader components. This is performed
from an application/project, by creating bootloader components. To do so,
it is required to create a `bootloader_component` directory containing
the custom modules to be compiled with the bootloader.

Thanks to this, two solutions are available to override the bootloader now:
- Using hooks within a user bootloader component
- Using a user defined `main` bootloader component to totally override the
  old implementation

Please check the two new examples in `examples/custom_bootloader`

* Closes https://github.com/espressif/esp-idf/issues/7043

examples/custom_bootloader/bootloader_hooks/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 3.5)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(main)

examples/custom_bootloader/bootloader_hooks/Makefile

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+
+#
+# This is a project Makefile. It is assumed the directory this Makefile resides
+# in is a project subdirectory.
+#
+
+PROJECT_NAME := bootloader_hooks
+
+include $(IDF_PATH)/make/project.mk

examples/custom_bootloader/bootloader_hooks/README.md

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+# Bootloader hooks
+
+(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 hooks to the 2nd stage bootloader.
+
+## 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 output should be as followed:
+```
+I (24) HOOK: This hook is called BEFORE bootloader initialization
+I (37) boot: [...]
+[...]
+I (60) HOOK: This hook is called AFTER bootloader initialization
+```
+
+And finally the application will start and show the message:
+```
+User application is loaded and running.
+```
+
+## Organisation of this example
+
+This project contains an application, in the `main` directory that represents a user program.
+It also contains a `bootloader_components` that, as it name states, contains a component compiled and linked with the bootloader.
+
+Below is a short explanation of files in the project folder.
+
+```
+├── CMakeLists.txt
+├── main
+│   ├── CMakeLists.txt
+│   └── main.c                 User application
+├── bootloader_components
+│   └── my_boot_hooks
+│       ├── CMakeLists.txt   
+│       └── hooks.c            Implementation of the hooks to execute on boot
+└── README.md                  This is the file you are currently reading
+```
+Bootloader hooks are **not** supported in legacy `make` build system. They are only supported with `CMake` build system.
+
+## Note about including weak symbols
+
+The components in ESP-IDF are compiled as static libraries. Moreover, the bootloaders' hooks are declared as `weak`. Thus, when
+defining hooks for the bootloader, we **must** tell the compiler to always include our library (`my_boot_hooks`) in the link process.
+To achieve this, we need to define an extra symbol: `bootloader_hooks_include`. In our case, this symbol is a function defined in
+`bootloader_components/my_boot_hooks/hooks.c`. This will make the linker include all the symbols contained in that file.

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

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+idf_component_register(SRCS "hooks.c")
+
+# 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_hooks/bootloader_components/my_boot_hooks/hooks.c

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+#include "esp_log.h"
+
+/* Function used to tell the linker to include this file
+ * with all its symbols.
+ */
+void bootloader_hooks_include(void){
+}
+
+
+void bootloader_before_init(void) {
+    /* Keep in my mind that a lot of functions cannot be called from here
+     * as system initialization has not been performed yet, including
+     * BSS, SPI flash, or memory protection. */
+    ESP_LOGI("HOOK", "This hook is called BEFORE bootloader initialization");
+}
+
+void bootloader_after_init(void) {
+    ESP_LOGI("HOOK", "This hook is called AFTER bootloader initialization");
+}

examples/custom_bootloader/bootloader_hooks/example_test.py

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+from __future__ import print_function
+
+import ttfw_idf
+
+
+@ttfw_idf.idf_example_test(env_tag='Example_GENERIC', target=['esp32', 'esp32s2', 'esp32c3'])
+def test_custom_bootloader_hooks_example(env, _):  # type: ignore
+    # Test with default build configurations
+    dut = env.get_dut('main', 'examples/custom_bootloader/bootloader_hooks')
+    dut.start_app()
+
+    # Expect to read both hooks messages
+    dut.expect('This hook is called BEFORE bootloader initialization')
+    dut.expect('This hook is called AFTER bootloader initialization')
+
+
+if __name__ == '__main__':
+    test_custom_bootloader_hooks_example()

examples/custom_bootloader/bootloader_hooks/main/CMakeLists.txt

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

examples/custom_bootloader/bootloader_hooks/main/bootloader_hooks_example_main.c

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+#include <stdio.h>
+
+void app_main(void)
+{
+    /**
+     * Nothing special is done here, everything interesting in this example
+     * is done in the custom bootloader code, located in:
+     * `bootloader_components/my_boot_hooks/hooks.c`
+     */
+    printf("User application is loaded and running.\n");
+}

examples/custom_bootloader/bootloader_hooks/main/component.mk

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+#
+# "main" pseudo-component makefile.
+#
+# (Uses default behaviour of compiling all source files in directory, adding 'include' to include path.)