Reven for ether_calls,v 1.1.

Some platforms have rules for retrieving the MAC address for an interface
beyond what properties exist. For example, a local address maybe be
present in a device tree property, but a system-wide property may indicate
that it should not be used (in favor of e.g. a singular system MAC addres -
LOOKIN' AT YOU, SUNW!).

So, the ether-get-mac-address device call is introduced to handle this
situation. Consult it before the standard properites, and if it succeeds,
use its result.