Added an interface for accessing the RXFIFO in a non-split phase (synchronous) way...

Added an interface for accessing the RXFIFO in a non-split phase (synchronous) way. The reason for having this interface is that, according to datasheet, before flushing the RXFIFO one has to read at least one byte from the RXFIFO and the radio driver code is less complicated if flushing the RXFIFO can happen in a non-split phase (synchronous) way.

This commit is in agreement with JeongGil Ko (John Ko), who is the current maintainer of the CC2420 stack.