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'd say the vtimer can live with less accurate values.
But if you insist, I'm fine with defining two values and have the vtimer-specific one defined automatically calculated based on the hwtimer value unless overwritten. There's not much to be gained though, as any value ends up being passed to hwtimer and converted to ticks anyways.