If we're netbooting on a system with, for example, an ISA DE204 Ethernet
interface, we don't have sufficient information to find the device using
the bus/slot scheme that we do with a PCI network interface.  However, in
these cases, some versions of the SRM console supply the MAC address of
the interface in the booted_dev environment variable, like so:

BOOTP 1 1 0 0 0 5 0 08-00-2B-xx-xx-xx 1

So, if we weren't able to find the booted device by the usual means,
check for this and, if we find a MAC address, try to find the network
interface by the MAC address.

(thorpej)