ath79: image: retire combined-image for Adtran/Bluesocket devices

ath79: image: retire combined-image for Adtran/Bluesocket devices

During review it slipped by that these devices use combined-image which
should never be used for newly added ones. Therefore switch to
sysupgrade-tar generated images introduced in 8f6f260 ("ath79:
routerstation: prepare to use sysupgrade-tar format image"). The
sysupgrade accepts both images for now so no reression should occur.

Cc: Brian Gonyer <bgonyer@gmail.com>
Cc: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>