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.

Missing:
- [ ] adopt tests
- [ ] find appropriate values for each board
Should I introduce a variable for this along with the PR?