ATSTK1002: Remove default ethernet addresses

Wolfgang is right: It's not a good idea to set up default initial
ethernet addresses for a board, even though they belong to the local
range.

This will change the failure mode from "IT manager screams at you for
using duplicate ethernet addresses" to a nice error message explaining
that the ethernet address hasn't been set properly.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>