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/example_test.py

@@ -0,0 +1,18 @@
+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()