Generic sniffer implementation.

A **sniffer** implementation is added in tools/sniffer.
It currently supports **mbxxx** and **sky** platforms. (The sniffing hook is implemented as a fake RDC driver, which makes it platform-independent. Thus very little adaptation is needed to support more platforms.)
Quickstart:
```
$ make TARGET=<target>                       # compilation
$ make TARGET=<target> sniffer.upload        # flashing
$ make TARGET=<target> sniff                 # on-line display (wireshark) 
```
You will find more details in Readme.txt, but do not hesitate to ask me if you need more info. Thanks!