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/drivers/px4fmu/fmu.cpp

@@ -1784,7 +1784,7 @@ fmu_main(int argc, char *argv[])
 	}
 
 	if (!strcmp(verb, "id")) {
-		char id[12];
+		uint8_t id[12];
 		(void)get_board_serial(id);
 
 		errx(0, "Board serial:\n %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X",