ar71xx-generic: add support for Carambola 2

Device information can be found at:
http://www.8devices.com/products/carambola-2
https://wiki.openwrt.org/toh/8devices/carambola2

I only did some minimal testing of gluon on the carambola 2 development
board:

 - Config mode works
 - Connects to Wifi Mesh
 - Allows clients to connect

Notably, autoupgrade has not yet been tested.

Change to 010-primary-mac is necessary as the mac address printed
on the sticker is the one of eth0, not the wifi mac.