[Tinyos Core WG] Small gotcha in UartStream interface

[Phil Buonadonna]

One of the objective of using UartStream was to enable DMA based data
transfers. 

However the 'receivedByte()' event conflicts with the ability to use DMA
based receives.  Specifically, there's no way to grab each individual byte
if a DMA is in progress.

I think the fix is just to change the 'semantics' of this functionality so
that it CAN'T be used in conjuction with the receive() call.  

Thoughts?

pb