core,sys,boards: introduce hwtimer_spin barrier

Introduce HWTIMER_SPIN_BARRIER (API change):
Boards need to 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`.

Values are based on the current implementation, suggestions welcome.

Based on #1407
I'll do a proper review (maybe tomorrow) and tell you if you can live the "low" accuracy. Chances are that you PR is good enough for the beginning.