mkimage.sh.in: compress resultant images

mkimage.sh.in

@@ -349,6 +349,8 @@ case "$PLATFORM" in
         info_msg "Sucessfully created ${FILENAME%.img}.tar.gz image."
         ;;
     *)
+        info_msg "Compressing $FILENAME with xz (level 9 compression)"
+        xz -T0 -9 "$FILENAME"
         info_msg "Successfully created $FILENAME image."
         ;;
 esac