feat(bootloader): add an example that implements multiboot

Add a new example for the bootloader that shows how to override it to implement multiboot.

examples/custom_bootloader/bootloader_multiboot/main/CMakeLists.txt

@@ -0,0 +1,3 @@
+idf_component_register(SRCS "bootloader_multiboot_example_main.c"
+                       INCLUDE_DIRS "."
+                       PRIV_REQUIRES newlib)

examples/custom_bootloader/bootloader_multiboot/main/bootloader_multiboot_example_main.c

@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: 2015-2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include <stdio.h>
+
+void app_main(void)
+{
+    /**
+     * Nothing special is done here, everything interesting in this example
+     * is done in the custom bootloader code, located in:
+     * `bootloader_components/main/bootloader_start.c`
+     */
+    printf("Application started!\n");
+}