mvebu: append metadata to clearfog sd card images

Fixes the following issue:

root@LEDE:/# sysupgrade /tmp/lede-mvebu-armada-388-clearfog-sdcard.img.gz
Saving metaconfig...
Image metadata not found
Use sysupgrade -F to override this check when downgrading or flashing to vendor firmware
Image check 'fwtool_check_image' failed.
root@LEDE:/#

Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

target/linux/mvebu/image/Makefile

@@ -176,7 +176,7 @@ define Device/armada-388-clearfog
   DEVICE_TITLE := SolidRun ClearFog
   DEVICE_PACKAGES := mkf2fs e2fsprogs swconfig kmod-fs-vfat kmod-nls-cp437 kmod-nls-iso8859-1
   IMAGES := sdcard.img.gz
-  IMAGE/sdcard.img.gz := boot-scr | boot-img | sdcard-img | gzip
+  IMAGE/sdcard.img.gz := boot-scr | boot-img | sdcard-img | gzip | append-metadata
   IMAGE_NAME = $$(IMAGE_PREFIX)-$$(2)
 endef
 TARGET_DEVICES += armada-388-clearfog