ar71xx: unbreak building images for TL-WR1041N-v2, TL-WR2543N

ar71xx: unbreak building images for TL-WR1041N-v2, TL-WR2543N

Commit 48788063266ad99bdab03b53cbe7e1a564c2d9b3 (r33251) introduced a
regression with board= kernel command line parameter incorrectly
initialised.

The faulty builds propogated to the snapshots download area. This makes
the device unbootable and TP-Link users have to attach the serial to
recover.

Here is the q&d way i used to identify MIPS boards that didn't have a
corresponding entry in the image/Makefile:

for i in `find ../files/arch/mips/ath79/ -type f -exec cat {} \; \
	| sed -n '/MIPS_MACHINE/ { s/^[^"]*"\([^"]*\)".*$/\1/; p; }'`; do
		grep -q ,$i, Makefile || echo $i;
done > missing-boards.new

Reported-by: Casper on IRC
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33274 3c298f89-4303-0410-b956-a3cf2f4a3e73