ramips: Archer C50v1: support US and EU versions

ramips: Archer C50v1: support US and EU versions

For the Archer C50v1, the EU and US versions are differentiated by their
respective HW additional version (0x0 for US, 0x2 for EU).

The stock web interface checks this field before flashing, making it
impossible to flash the current (US) factory image on EU hardware.

However the bootloader does not check this field, making it possible to use
a single sysupgrade image for both hardware.

This patch adds the necessary build bits to generate both EU and US factory
images, and renames the target as "Archer C50v1" since there are as of now
3 different versions of Archer C50 (all with different CPUs).

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>