ramips: mt7620: 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.

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