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:
- [x] adopt tests
- [x] find appropriate values for each board (invalid as per https://github.com/RIOT-OS/RIOT/pull/1409#issuecomment-49262386)

Bonus: clean up long lines
IMHO, yes. Then users can fine-tune the value for specific boards without touching hwtimer code.