ramips: mt76x8: use flash location for wan_mac in 02_network

ramips: mt76x8: use flash location for wan_mac in 02_network

This uses the flash locations instead of eth0 MAC address to
calculate MAC address increments for WAN.

The change will make the MAC address setup of a particular device
more obvious and removes the dependency of 02_network on the eth0
initialization.

While at it, change the partition label for zyxel,keenetic-extra-ii
to factory to be consistent with node label and all the other devices.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>