Add a developer tool: generate random MAC address.

This might be handy for creating a new MAC address quickly (think of MAC address conflicts, since proadhoc doesn't like the same MAC addresses, iirc). It just reuses the func in Config.cpp.

On that note, we could probably remove the change mac address screen stuff too, if we can just use this. There's little reason to edit it by hand. In that case though, I could simply move this into the other option's current place..