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
core/hwtimer: add HWTIMER_WAIT_OVERHEAD
Merge pull request #1409 from LudwigOrtmann/hwtimer_spin-barrier

core,sys,boards: introduce hwtimer_spin barrier