UsbSerialPort: add purgeHwBuffers method.

Consolidates following changes from Felix (newest first):
  1123807 Rename flushHwBuffers to purgeHwBuffers
  3eb145d Use UsbSerialPort instead of UsbSerialDriver in SerialInputOutputManager
  f91a974 Return true in flushHwBuffers default implementation if there is nothing to flush
  69c0b59 Implement flushHwBuffers for Cp2102 driver
  4a41bd9 Rename UsbSerialPort.flush function to flushHwBuffers
  c908da4 Refactoring: Make ProlificSerialDriver a subclass of CdcAcmSerialDriver
  39cb480 Refactoring: New UsbSerialPort interface
  d542f64 Refactoring: Do not require permission to USB device when probing
  9a13571 Support flushing non-written / non-read data
Classpath update from ADT.
Adding support for the new FT231X IC from FTDI

To use it as a serial-USB bridge it's the same thing as a FT232 IC