mediatek: split base-files into subtargets

This splits some base-files across subtargets, as done previously
on ath79 and ramips and also introduced for mt7629 subtarget here
already. Most of the existing base-files content is specific to
mt7623.

While at it, apply the following fixes:
- Remove lots of trailing whitespaces
- Remove wildcard on unielec,u7623-02-emmc-512m
- Remove inconsistent quotation marks in cases

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Acked-by: John Crispin <john@phrozen.org>

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

@@ -0,0 +1,26 @@
+platform_do_upgrade() {
+	local board=$(board_name)
+
+	case "$board" in
+	*)
+		default_do_upgrade "$1"
+		;;
+	esac
+}
+
+PART_NAME=firmware
+
+platform_check_image() {
+	local board=$(board_name)
+
+	[ "$#" -gt 1 ] && return 1
+
+	case "$board" in
+	*)
+		echo "Sysupgrade is not supported on your board yet."
+		return 1
+		;;
+	esac
+
+	return 0
+}