Both Build/sq201-images and Build/nas4220b-images scripts
are very similar. This patch unifies both methods at the
cost of renaming the produced sysupgrade file names, but
with the benifit of creating better reproducible files.
The patch also fixes a race in parallel builds in which case
the ImageInfo of one device could end up in both sysupgrade
files.
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
