[netdevice] Add "bustype" and "busloc" settings

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[settings] Add "busdevfn" setting type

Allow network device's "busloc" setting to be formatted as a PCI
bus:dev.fn address using e.g. ${net0/busloc:busdevfn}.

Signed-off-by: Michael Brown <mcb30@ipxe.org>