Clean up setting channel and CCA-threshold for CC2420 based motes.
The CCA-threshold now defaults to -46 which give better simulation
results and typically also better experimental results.
This adjustment is also needed due to commit 0a13f99 in mspsim. As
promised in https://github.com/mspsim/mspsim/pull/18 it broke the
regression tests.
Added mspsim as a submodule instead of as a binary mspsim.jar file