ar71xx: build firmware image for the WNDR3700 v4 board

Based on the the WNDR3700v4 support patch from Ralph Perlich:
  http://patchwork.openwrt.org/patch/4763/

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40481 3c298f89-4303-0410-b956-a3cf2f4a3e73