Adding CC2420 support to TOSSIM. 

These set of changes add CC2420 support to TOSSIM. Now code that uses CC2420's features like Snoop, getRSSI, setChannel etc can be simulated under TOSSIM. The changes are adapted from http://hinrg.cs.jhu.edu/git/?p=mike/tinyos-2.x.git and ported to the latest TinyOS.