ath79: restore sysupgrade support for ja76pf2 and routerstations

ath79: restore sysupgrade support for ja76pf2 and routerstations

Because the bug described in FS#2428 has been fixed with bf2870c1d9e1
("kernel: fix mtd partition erase < parent_erasesize writes") these
devices can now safely do sysupgrade.

Restore sysupgrade support disabled in:

0cc87b3bacee ("ath79: image: disable sysupgrade images for routerstations
and ja76pf2")

cc5256a8bfa0 ("ath79: base-files: disable sysupgrade for routerstations
and ja76pf2")

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>
[move Build block, remove check-size argument, wrap sysupgrade line,
make commit message easier to read]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>