ar71xx: enable images for WD My Net N750

Now that the switch works correctly we can enable the images for this
board.

Flashing from the factory firmware, the factory failsafe loader and
sysupgrade was tested successfully.

Signed-off-by: Felix Kaechele <heffer@fedoraproject.org>
Patchwork: http://patchwork.openwrt.org/patch/4617/


Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39130