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/main/CMakeLists.txt

@@ -0,0 +1,2 @@
+idf_component_register(SRCS "bootloader_hooks_example_main.c"
+                    INCLUDE_DIRS ".")