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

components/bootloader/project_include.cmake

@@ -117,6 +117,7 @@ externalproject_add(bootloader
     CMAKE_ARGS  -DSDKCONFIG=${sdkconfig} -DIDF_PATH=${idf_path} -DIDF_TARGET=${idf_target}
                 -DPYTHON_DEPS_CHECKED=1 -DPYTHON=${python}
                 -DEXTRA_COMPONENT_DIRS=${CMAKE_CURRENT_LIST_DIR}
+                -DPROJECT_SOURCE_DIR=${PROJECT_SOURCE_DIR}
                 ${sign_key_arg} ${ver_key_arg}
                 # LEGACY_INCLUDE_COMMON_HEADERS has to be passed in via cache variable since
                 # the bootloader common component requirements depends on this and