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/subproject/main/bootloader_hooks.h

@@ -0,0 +1,38 @@
+// Copyright 2015-2021 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef BOOTLOADER_HOOKS_H
+#define BOOTLOADER_HOOKS_H
+
+/**
+ * @file The 2nd stage bootloader can be overriden or completed by an application.
+ * The functions declared here are weak, and thus, are meant to be defined by a user
+ * project, if required.
+ * Please check `custom_bootloader` ESP-IDF examples for more details about this feature.
+ */
+
+
+/**
+ * @brief Function executed *before* the second stage bootloader initialization,
+ * if provided.
+ */
+void __attribute__((weak)) bootloader_before_init(void);
+
+/**
+ * @brief Function executed *after* the second stage bootloader initialization,
+ * if provided.
+ */
+void __attribute__((weak)) bootloader_after_init(void);
+
+#endif // BOOTLOADER_HOOKS_H