ramips: add eth0 MAC address for Edimax RG21S

ramips: add eth0 MAC address for Edimax RG21S

So far, lan/wan MAC address for Edimax RG21S are only read using
mtd_get_mac_ascii, so eth0.1 and eth0.2 addresses are set, but
eth0 address is random. Since the device's LAN address is the same
as for 2.4 GHz, though, this patch set's the eth0 address based
on the 2.4 GHz one, which can be extracted by mtd-mac-address.

This will also allow to move the label MAC address setup to DT.

The setup of lan_mac and wan_mac are kept in 02_network, so those
locations are still in use, too.

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