On a fast CPU with a slow timer (e.g. XTIMER_SHIFT > 0) it is possible
that now == _xtimer_now() when spinning for the overflow. In the extreme
case When this happens _next_period() will be called more than once
until the timer overflows for real.
Fault observed in real life when running on a 32.768 kHz timer on a
~96 MHz clocked mulle (Kinetis K60, Cortex-M4). _next_period() was
called 9 times during the same ISR call before the 32 kHz timer
overflowed.
- more robust underflow protection in xtimer_usleep_until()
- use relative target in xtimer_spin()
- honour reference in isr when spinning until timer target
- add XTIMER_BACKOFF to xtimer_spin_until() target when backing off in
_timer_set_absolute()
- doxygen updates
