mediatek: mt7622: validate uImage.FIT

Validate the uImage.FIT instead of only checking the image magic.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/mt7622/base-files/lib/upgrade/platform.sh

@@ -69,11 +69,8 @@ platform_check_image() {
 		return $?
 		;;
 	*)
-		[ "$magic" != "d00dfeed" ] && {
-			echo "Invalid image type."
-			return 1
-		}
-		return 0
+		fit_check_image "$1"
+		return $?
 		;;
 	esac