tests: let license checker check headers, too
core: clist is a cyclic list
core: clist without explicit thread

Right now the core component `clist` is a generic cyclic doubly-linked list.
In the core it is used in `tcb_t::rq_entry`.
Further it is used `net_if.c`.

This commit removes the member `clist_node_t::data` which stored the
pointer to the `tcb_t` instance of which the clist is already a member.
The needless member added `sizeof (int)` bytes to every instance of
`tcb_t`.

In `net_if.c` the clist was used in a type-punned way, so that the
change won't affect it.
Merge pull request #1531 from LudwigOrtmann/header_license_headers

tests: let license checker check headers, too
Merge pull request #1013 from Kijewski/clist_wo_thread

 core: clist without explicit thread
introduce HWTIMER_SPIN_BARRIER (API change)

Boards should define HWTIMER_SPIN_BARRIER that is used to decide
whether it makes sense to set a timer and yield or call hwtimer_spin
instead.
Used by `core/hwtimer.c` and `sys/vtimer/vtimer.c`.
A default value is provided and a warning is printed when it is used.
core/hwtimer: clean up header

- break/shorten overlong lines
- improve grammar
tests: adopt for hwtimer_spin_barrier

also clean out unneeded headers