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_start.c

@@ -16,6 +16,7 @@
 #include "bootloader_init.h"
 #include "bootloader_utility.h"
 #include "bootloader_common.h"
+#include "bootloader_hooks.h"
 
 static const char *TAG = "boot";
 
@@ -29,11 +30,21 @@ static int selected_boot_partition(const bootloader_state_t *bs);
  */
 void __attribute__((noreturn)) call_start_cpu0(void)
 {
+    // (0. Call the before-init hook, if available)
+    if (bootloader_before_init) {
+        bootloader_before_init();
+    }
+
     // 1. Hardware initialization
     if (bootloader_init() != ESP_OK) {
         bootloader_reset();
     }
 
+    // (1.1 Call the after-init hook, if available)
+    if (bootloader_after_init) {
+        bootloader_after_init();
+    }
+
 #ifdef CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP
     // If this boot is a wake up from the deep sleep then go to the short way,
     // try to load the application which worked before deep sleep.