ar71xx: generate US- and EU-specific images for the Archer C7 v2

TP-Link has started providing US- and EU-specific stock firmwares that only
allow upgrading to firmwares with the same region code. Provide factory
images for both these regions.

To avoid confusing users outside these regions, we still provide a
"universal" factory.bin without a region code, although flashing either of
the US and EU images would work as well.

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