treewide: call check-size before append-metadata

treewide: call check-size before append-metadata

sysupgrade metadata is not flashed to the device, so check-size
should be called _before_ adding metadata to the image.

While at it, do some obvious wrapping improvements.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Acked-by: Paul Spooren <mail@aparcar.org>