- Safer handling of targets when using XTIMER_MASK
 - Reduce number of redundant branches
 - Avoid possible timer underflow when subtracting XTIMER_OVERHEAD
 - Spin at the end if target time is close

…CKOFF

If XTIMER_OVERHEAD is greater than either XTIMER_BACKOFF and/or
XTIMER_ISR_BACKOFF then it will be possible schedule low level timers
with a negative offset, which means the timer will delay until the next
overflow.

64-bit version of div_u32_by_15625div512

also updated tests-div to use correct order for expected and actual values in unit tests (embUnit)

Set XTIMER_HZ in periph_conf.h to specify the xtimer hardware timer
frequency.

The xtimer system uses the hardware timer's own tick representation
internally but a thin wrapper layer is added as a shim between xtimer
core and the public xtimer API which is based around microseconds as
the time unit.

Initially supports only 1000000 Hz (and powers-of-two divisions of it)
and 32768 Hz, but can easily be expanded by adding a new case for
XTIMER_HZ inside xtimer.h

 - drift is the total difference from the expected time
 - jitter is the difference only since the last printout

… timers, and the 2**32 usec barrier