ar71xx: fix model string detection on NETGEAR WNDR3700/3800/WNDRMAC

ar71xx: fix model string detection on NETGEAR WNDR3700/3800/WNDRMAC

There were a few issues with the existing code to detect the model string:
* Always using the string starting with byte 56 would cut off the W of WNDR when
  the ID starts with 29763654+16+64 instead of 29763654+16+128
* The string contained garbage after the zero byte instead of cutting it off
  after the zero (which wasn't always visible using busybox tools, but could
  confuse other scripts)

Tested on a WNDR3700v1 and a WNDR3700v2 using the new 29763654+16+64 ID in the
ART. Furthermore, tested against ART dumps of a WNDR3700v2 using the old
$'\xff...' value and a WNDR3800.

The [ -z "$model" ] check was dropped as there is no way to actually hit this
unless no ART partition is found at all.

The awk command was carefully crafted to work both with gawk and the (horribly
broken) busybox awk.

Fixes #18992.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45140 3c298f89-4303-0410-b956-a3cf2f4a3e73