A lot of these embedded boards don't have a unique MAC address per
device stored somewhere unique - sometimes they'll have one MAC
for both arge NICs; someties they'll have one MAC for both arge NICs
/and/ the ath NICs. In these instances, we need to derive device
specific MAC addresses from the base MAC address.
These functions will be used by some follow-up code that'll slot
into if_arge and if_ath.