ar71xx: fix wndr3700_board_detect for some NETGEAR WNDR3700v2

There are a few NETGEAR devices which don't terminate the model name in the
ART with a NUL byte, at least some NETGEAR WNDR3700v2. The current awk
expression doesn't match 0xFF bytes, so AR71XX_MODEL contains lots of
trailing 0xFF garbage in this case.

Fix this by matching for the first non-printable character and explicitly
setting LC_CTYPE=C (probably not strictly necessary on OpenWrt, but will
definitely work like this, even when awk supports locales and LANG is set).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46455 3c298f89-4303-0410-b956-a3cf2f4a3e73