ar71xx: fix NetGear initramfs image names

The '-initramfs' string is duplicated in the image names,
fix it.

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

git-svn-id: svn://svn.openwrt.org/openwrt@37892 3c298f89-4303-0410-b956-a3cf2f4a3e73