Support PiZero (rpiboot + Ethernet-gadget mode)

[macmpi]

One can boot PiZero without SD, with rpiboot and then kicking-up a NFS server over Ethernet-gadget mode.

Manual setup is a bit complex, and it would be nice if piserver could support that second-stage part (DHCP/NFS server), so that one can just run rpiboot and piserver to get such config running.

That'll make highly cost-efficient tiny PiZero environment to experiment hacking and development.

Thanks for consideration.