Add serial.{c,h} from tankbot project
Replace libftdi/ftd2xx with native serial port code.

This has two benefits:
 - No need to install ftd2xx library on windows
   (only need the drivers, which are easy to install)
 - No need to unplug/replug when to change device association
   from libusb to emulated serial before doing a firmware
   upgrade on linux.