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>

SVN-Revision: 47270