type process_data_t. This was an artifact when the choice was
made to use the void * type for the data parameter in processes.
Changed parameter 'void * data' of process_post_synch to
process_data_t for consistency.
Checked all the uses of process_start() in contiki and fixed casts
of the data parameter.
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.
