add UartByte.{send,receive}Avail definitions for UartByte interface
msp430/x5/usci-v2: cleanup Msp430UsciUartP

- clean up comments
- replace various register access with direct calls for the function
  Usci.setIe(UCRXIE) -> Usci.enableRxIntr
- reorganize some of the code.  Easier to read.