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_override/bootloader_components/main/CMakeLists.txt

@@ -0,0 +1,9 @@
+idf_component_register(SRCS "bootloader_start.c"
+                    REQUIRES bootloader bootloader_support)
+
+idf_build_get_property(target IDF_TARGET)
+# Use the linker script files from the actual bootloader
+set(scripts "${IDF_PATH}/components/bootloader/subproject/main/ld/${target}/bootloader.ld"
+            "${IDF_PATH}/components/bootloader/subproject/main/ld/${target}/bootloader.rom.ld")
+
+target_linker_script(${COMPONENT_LIB} INTERFACE "${scripts}")

examples/custom_bootloader/bootloader_override/bootloader_components/main/bootloader_start.c

@@ -0,0 +1,67 @@
+/*
+ * SPDX-FileCopyrightText: 2015-2021 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include <stdbool.h>
+#include "esp_log.h"
+#include "bootloader_init.h"
+#include "bootloader_utility.h"
+#include "bootloader_common.h"
+
+static const char* TAG = "boot";
+
+static int select_partition_number(bootloader_state_t *bs);
+
+/*
+ * We arrive here after the ROM bootloader finished loading this second stage bootloader from flash.
+ * The hardware is mostly uninitialized, flash cache is down and the app CPU is in reset.
+ * We do have a stack, so we can do the initialization in C.
+ */
+void __attribute__((noreturn)) call_start_cpu0(void)
+{
+    // 1. Hardware initialization
+    if (bootloader_init() != ESP_OK) {
+        bootloader_reset();
+    }
+
+#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.
+    // It skips a lot of checks due to it was done before (while first boot).
+    bootloader_utility_load_boot_image_from_deep_sleep();
+    // If it is not successful try to load an application as usual.
+#endif
+
+    // 2. Select the number of boot partition
+    bootloader_state_t bs = {0};
+    int boot_index = select_partition_number(&bs);
+    if (boot_index == INVALID_INDEX) {
+        bootloader_reset();
+    }
+
+    // 2.1 Print a custom message!
+    esp_rom_printf("[%s] Custom bootloader has been initialized correctly.\n", TAG);
+
+    // 3. Load the app image for booting
+    bootloader_utility_load_boot_image(&bs, boot_index);
+}
+
+// Select the number of boot partition
+static int select_partition_number(bootloader_state_t *bs)
+{
+    // 1. Load partition table
+    if (!bootloader_utility_load_partition_table(bs)) {
+        ESP_LOGE(TAG, "load partition table error!");
+        return INVALID_INDEX;
+    }
+
+    // 2. Select the number of boot partition
+    return bootloader_utility_get_selected_boot_partition(bs);
+}
+
+// Return global reent struct if any newlib functions are linked to bootloader
+struct _reent *__getreent(void)
+{
+    return _GLOBAL_REENT;
+}