net-genrules: accept MAC address for $iface, cleanups

net-genrules: accept MAC address for $iface, cleanups

A MAC address is a unique identifier for a particular network interface.

We can use the MAC to generate udev rules to bring up that interface,
like we currently do with BOOTIF.

This patch allows interfaces to be specified as a MAC address, either
in the usual colon-separated form or the PXE-style dash-separated form.

(The latter is more useful on the commandline, since it allows for
arguments like: "ip=77-77-6f-6f-64-73:dhcp")

This is useful since it's common for a user who is booting a new OS for
the first time to know the MAC of the device, but not know what the
kernel name will be.