mediatek: add mt7629 subtarget with rfb image

base-files are added into subtarget directory like what's done
recently in ath79. For this subtarget, metadata checks are enforced
and a SUPPORTED_DEVICE is added to generate proper metadata.
Since we only have mt7629 support in 4.19, override KERNEL_PATCHVER
in target.mk for now.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/mediatek/mt7629/base-files/lib/upgrade/platform.sh

@@ -0,0 +1,16 @@
+PART_NAME=firmware
+REQUIRE_IMAGE_METADATA=1
+
+platform_check_image() {
+	return 0
+}
+
+platform_do_upgrade() {
+	local board=$(board_name)
+
+	case "$board" in
+	*)
+		default_do_upgrade "$1"
+		;;
+	esac
+}