Reduce code repetition for sd browser firmware update handling, add some .frsk entries (#543)

* Hacky port of FRSKY_FIRMWARE_EXT update menu
More work is needed to integrate/test the remaining menu items
May just ba a matter of correct parameters for deviceFirmwareUpdate

* Functionalise, of sorts

* Formatting

* Functionalise more repeated update code

radio/src/io/multi_firmware_update.h

@@ -104,7 +104,7 @@ class MultiFirmwareInformation
     const char * readV2Signature(const char * buffer);
 };
 
-enum MultiModuleType
+enum MultiModuleType : short
 {
   MULTI_TYPE_MULTIMODULE = 0,
   MULTI_TYPE_ELRS,