ath79: specify N and ND subversions of TL-WR941 with ALT0_MODEL

ath79: specify N and ND subversions of TL-WR941 with ALT0_MODEL

TP-Link's TL-WR941 is sold with detachable antennas
internationally (ND version), but with fixed antennas in China
(N version). While hardware and images are similar for both
variants of v2 and v4, they are different for v6.

Having both explicitly will make it easier for user to identify
the correct image, and most importantly will raise awareness
that N and ND are not necessarily always the same as for
TL-WR841 series.

With an image selection webpage, using ALT0_MODEL as in this
patch will provide the option to list versions for N and ND
separately.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>