ar71xx: image: allow to use optional parameters for MkuImageLzma/initramfs

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38658 3c298f89-4303-0410-b956-a3cf2f4a3e73
ar71xx: image: fix WNDR4300 initramfs image generation

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38659 3c298f89-4303-0410-b956-a3cf2f4a3e73