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.
declarations of functions for setting and getting a node ID number, a
functionality that exists on many platforms. Since this functionality
was not considered part of the Contiki core, each platform defined its
own node-id.h file. This commit attempts to clean this up by
collecting the node-id.h into a core/sys/node-id.h file that replaces
the old node-id.h files from the platform directories.
