ar71xx: remove fs_* variables from image/Makefile

Those are not needed since we are generating the
initramfs images along with the others.

Remove the variables and use the template names
directly.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37876 3c298f89-4303-0410-b956-a3cf2f4a3e73