[gnea/grbl-Mega PR#109] Made grbl serial port selectable in config.h

If you have a USB to serial (HC04) or Wifi (ESP8266) to serial adapter connected to your serial port, you have to disconnect this adapter before flashing GRBL. To avoid this, connect the adapter to USART 1,2 or 3 and change the serial port in config.h. You can than flash using the USB port connected to USART0 and communicate to grbl by an other serial port.