ar71xx: fix lzma-loader kernel command line passing

The generic kernel cmdline parser ignores argv[0], this caused a
regression for all lzma-loader based boards with linux 4.1

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46544 3c298f89-4303-0410-b956-a3cf2f4a3e73