ramips: rt305x: 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.

This removes the wan_mac setup for the following devices as they
do not set up a MAC address for ethernet in the first place:
- asiarf,awapn2403
- belkin,f7c027
- dlink,dir-615-d
- mofinetwork,mofi3500-3gn
- prolink,pwh2004
- ralink,v22rw-2x2
- unbranded,wr512-3gn-4m
- unbranded,wr512-3gn-8m

While at it, make some DT node labels consistent with the label
property.

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