pru UartIO: support discarding received data
pru UartIO: support discarding received data

- new method: discard()
- new optional parameter discard=False for write() and flush()
- new optional parameter flush=False for write()
- fixed bug in flush

discarding data will also ignore receive errors

mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio