made compilation of radio drivers mandatory even if not used (will not be included by linker anyway)
added java jar's to the git ignore list
removed two .jar-files that should not be part of the repo; they are re-created when starting Cooja anyway
renamed the channel clear function (prev CCA) for clarity
bugfix: should return statuscode, not plain number (thus always signalling error)
more sensible CCA (still buggy though) and some sane defaults