ar71xx: makefile changes to build image for MR3420v2

Patch-by: Dmytro <dioptimizer@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/3440/


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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35964 3c298f89-4303-0410-b956-a3cf2f4a3e73