Allow the kernel to recognize that it has been netbooted and to add the

Allow the kernel to recognize that it has been netbooted and to add the
boot interface to the "netboot" group.  efiboot grabs the MAC address
from the PXE environment, passes it to the kernel, where it is matched
against the list of ethernet interfaces and the boot device is set.
Concept and most of the code cribbed from amd64.
ok kettenis@