fix(bootloader): add legacy retained memory CRC calculation

* Closes https://github.com/espressif/esp-idf/issues/12849

In former versions of ESP-IDF, the user custom memory data in the retained memory
was taken into account during the CRC calculation. This was changed in a later
commit, the custom memory was ignored, therefore this can seen as a breaking change.
This commit gives the possibility to choose between the former (legacy) or new way of calculating the CRC.

components/bootloader/Kconfig.projbuild

@@ -383,6 +383,21 @@ menu "Bootloader config"
             (The application and bootoloader must be compiled with the same option).
             The RTC FAST memory has access only through PRO_CPU.
 
+    config BOOTLOADER_CUSTOM_RESERVE_RTC_IN_CRC
+        bool "Include custom memory in the CRC calculation"
+        depends on BOOTLOADER_CUSTOM_RESERVE_RTC
+        default n
+        help
+            This option allows the customer to use the legacy bootloader behavior when the
+            RTC FAST memory CRC calculation takes place. When this option is enabled, the
+            allocated user custom data will be taken into account in the CRC calculcation.
+            This means that any change to the custom data would need a CRC update to prevent
+            the bootloader from marking this data as corrupted.
+            If this option is disabled, the custom data will not be taken into account when
+            calculating the RTC FAST memory CRC. The user custom data can be changed freely,
+            without the need to update the CRC.
+            THIS OPTION MUST BE THE SAME FOR BOTH THE BOOTLOADER AND THE APPLICATION BUILDS.
+
     config BOOTLOADER_CUSTOM_RESERVE_RTC_SIZE
         hex "Size in bytes for custom purposes"
         default 0

components/bootloader_support/src/bootloader_common_loader.c

@@ -168,7 +168,7 @@ SOC_RESERVE_MEMORY_REGION(RTC_RETAIN_MEM_ADDR, RTC_RETAIN_MEM_ADDR + sizeof(rtc_
 #endif
 
 static uint32_t rtc_retain_mem_size(void) {
-#ifdef CONFIG_BOOTLOADER_CUSTOM_RESERVE_RTC
+#if CONFIG_BOOTLOADER_CUSTOM_RESERVE_RTC && !CONFIG_BOOTLOADER_CUSTOM_RESERVE_RTC_IN_CRC
     /* A custom memory has been reserved by the user, do not consider this memory into CRC calculation as it may change without
      * the have the user updating the CRC. Return the offset of the custom field, which is equivalent to size of the structure
      * minus the size of everything after (including) `custom` */