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 think the boards should directly define HWTIMER_SPIN_BARRIER in terms of microseconds. Every conversion will only reduce the accuracy of the value.