clean up mfu device side code

+ add xor calc to util (prep for desfire)
commented out MifareUWriteBlockCompat as it isn't used in client
currently (it is a command we could support..  but why?)
relabeled a few device side mfu functions to be clearer.

client/util.h

@@ -63,3 +63,5 @@ void binarraytobinstring(char *target,  char *source,  int length);
 uint8_t GetParity( char *string, uint8_t type,  int length);
 void wiegand_add_parity(char *target, char *source, char length);
 
+void xor(unsigned char *dst, unsigned char *src, size_t len);
+int32_t le24toh(uint8_t data[3]);