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>