net: Remove all references to CONFIG_ETHADDR and friends

We really don't want boards defining fixed MAC addresses in their config
so we just remove the option to set it in a fixed way. If you must have
a MAC address that was not provisioned, then use the random MAC address
functionality.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>