MAGIC value for powerReason should be stored in RTC backup on Horus (#7096)

- Allows the radio main application to start correctly when coming from an app in RAM
- Refactoring of the `UNEXPECTED_SHUTDOWN()` process
- Saves the RTC battery as BACKUP RAM is not needed on normal shutdown
- On PWR OFF on an ALERT, the radio won't show the next alert before shutting down
- Bootloader flashable through the SD Manager

radio/src/gui/480x272/radio_sdmanager.cpp

@@ -159,6 +159,10 @@ void onSdManagerMenu(const char * result)
     getSelectionFullPath(lfn);
     pushMenuTextView(lfn);
   }
+  else if (result == STR_FLASH_BOOTLOADER) {
+    getSelectionFullPath(lfn);
+    bootloaderFlash(lfn);
+  }
   else if (result == STR_FLASH_INTERNAL_MODULE) {
     getSelectionFullPath(lfn);
     FrskyDeviceFirmwareUpdate device(INTERNAL_MODULE);
@@ -278,6 +282,13 @@ bool menuRadioSdManager(event_t _event)
               POPUP_MENU_ADD_ITEM(STR_ASSIGN_SPLASH);
             }
           }
+          if (!READ_ONLY() && !strcasecmp(ext, FIRMWARE_EXT)) {
+            TCHAR lfn[_MAX_LFN+1];
+            getSelectionFullPath(lfn);
+            if (isBootloader(lfn)) {
+              POPUP_MENU_ADD_ITEM(STR_FLASH_BOOTLOADER);
+            }
+          }
           else if (!READ_ONLY() && !strcasecmp(ext, SPORT_FIRMWARE_EXT)) {
             if (HAS_SPORT_UPDATE_CONNECTOR())
               POPUP_MENU_ADD_ITEM(STR_FLASH_EXTERNAL_DEVICE);