First cut at polymorphic serial port implementation.  Split serialPort_t into uartPort_t and serialPort_t.  Calls to uartWrite() can now be replaced with calls to serialWrite().
Replacing calls to serialWriteByte(softSerial_t*, char) with calls to serialWrite(serialPort_t*, char).  This completes the proof of concept for polymorphic serial port implementations (uartPort and softSerialPort).