ar71xx: build TP-LINK Archer C7 images with US region code

ar71xx: build TP-LINK Archer C7 images with US region code

Since December 2015, the Archer C7 comes with an US-specific firmware. This
firmware will only allow upgrades with images that contain the US region
code in the firmware header.

The "universal" firmware distributed in the rest of the world doesn't care
about the region code, so we can just unconditionally set the region to US
for now. When other region-specific firmwares appear, we'll need to create
a factory image for each of them.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49217 3c298f89-4303-0410-b956-a3cf2f4a3e73