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.
Added compilation parameter "nodemac=1234" that can be used
to overwrite the MAC at compilation time, and that combined
with the burn-nodeid app can record a prestablished MAC on ext. mem.
