ar71xx: Add profile and build image for TP-Link MR12U v1

ar71xx: Add profile and build image for TP-Link MR12U v1

This patch adds corrected profile and build image for the TL-MR12U board.

Unfortunately i cannot test the factory image since i can't find anywhere
the original chinese firmware but
the generated openwrt-ar71xx-generic-tl-mr12u-v1-squashfs-sysupgrade.bin
worked just fine over a modified mr3040 factory image i managed to modify
to initially flash the mr12u from the original firmware.

Proper kernel support probably should be added since cat /proc/cpuinfo shows
MR13U.

Signed-off-by: Daniel Petre <daniel.petre@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44427 3c298f89-4303-0410-b956-a3cf2f4a3e73