Remove the "Change MAC Address" screen, and generate a random one instead.

~~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..~~

There's no point editing it by hand when we can simply push one button to generate a new one. If it's *really* that much of a problem to require a specific one, a trek through ppsspp.ini isn't that hard.