ramips: initialize MAC addresses from flash where possible

This patch changes wan MAC address setup from retrieving it by
calculation to reading it from flash.

Changes are limited to cases where on-device check was possible.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[fix mac for newifi-d1; drop adslr,g7 because it's unlikely for
vendor to specifically use 2.4g mac as wan_mac]
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>