ar71xx: build image for the TP-LINK TL-WR941N v6 board

TL-WR941N v6 use almost same IC as WDR3500.

[juhosg: change subject, and change fw layout from 4M to 4Mlzma]

Patchwork: http://patchwork.openwrt.org/patch/3719/
Signed-off-by: ShuYu Wang <andelf@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37471 3c298f89-4303-0410-b956-a3cf2f4a3e73