tests: add pattern for short lgplv2.1 header

Add the license checker pattern which is advertised in our wiki.
Merge pull request #1396 from LudwigOrtmann/lgplv2.1-short

tests: add pattern for short lgplv2.1 header
core/thread: remove thread_create_arg from header

It's just a leftover from #856
Merge pull request #1397 from LudwigOrtmann/thread_fixup

core/thread: remove thread_create_arg from header
sys/transceiver: fix t9r_register API compliance
sys/transceiver: implement transceiver_unregister
sys/transceiver: refactor un/register

remove race conditions
make less restrictive (allow double-un/registers)
make loops more readable
sys/transceiver: use size_t instead of u8 in loops