ar71xx: enable building ramdisk images for WZRHPG30XNH board types

Signed-off-by: Luka Perkov <luka@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35874 3c298f89-4303-0410-b956-a3cf2f4a3e73