board_serial: use a uint8_t buffer

we should not be using 'char' for binary APIs, as the C standard does
not specify if it is signed or unsigned, so results may not be
consistent

src/modules/systemlib/otp.h

@@ -125,7 +125,7 @@ struct otp_lock {
 #pragma pack(push, 1)
 union udid {
 	uint32_t	serial[3];
-	char  data[12];
+	uint8_t  data[12];
 };
 #pragma pack(pop)