gemini: Support sysupgrade on DIR-685

This makes sysupgrade work on the D-Link DIR-685 after
initial factory install.

We create the platform.sh script to support sysupgrade
on more targets as we move on with sysupgrade support.

Cc: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
[cleanup in platform.sh, removed superfluous SUPPORTED_DEVICES]

target/linux/gemini/base-files/lib/upgrade/platform.sh

@@ -0,0 +1,25 @@
+REQUIRE_IMAGE_METADATA=1
+
+platform_check_image() {
+	local board=$(board_name)
+
+	case "$board" in
+	dlink,dir-685)
+		return 0
+		;;
+	esac
+
+	echo "Sysupgrade is not yet supported on $board."
+	return 1
+}
+
+platform_do_upgrade() {
+	local board=$(board_name)
+
+	case "$board" in
+	dlink,dir-685)
+		PART_NAME=firmware
+		default_do_upgrade "$ARGV"
+		;;
+	esac
+}