kernel: Fix environment pointer setup in ar71xx/ath79

kernel: Fix environment pointer setup in ar71xx/ath79

Observed on ar71xx/ath79 platforms such as Ubiquiti RouterStations.Reported in #20642.:
(https://dev.openwrt.org/ticket/20642).

If embedded command-line text exists with CONFIG_IMAGE_CMDLINE_HACK=y,firmware
init doesn't initialize environment pointer (fw_init_cmdline not called).

arcs_cmdline is not initialized before calling strlcat.

Signed-off-by: Ted Hess <thess@kitschensync.net>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47270 3c298f89-4303-0410-b956-a3cf2f4a3e73