fix native hwtimer

The hardware timer used relative offsets that were never updated
before. This leads to two problems:
a) later timers will get pushed into the future by the amount of
   previous timers
b) if a short timer is set continuously, a longer timer will never be
   called

Example:
a)
Timer a with 500 ms is set, timer b with 600 ms is set.
timer a expires after 500 ms, timer b will be set to expire in 600 ms
which totals to 1100 ms.
b)
Timer a is set to 500 ms, timer b is set to 600 ms.
Timer a expires and is set again. Now timer a will expire in 500 ms
and timer b will be pushed further into the future. Repeating this
will lead to b never expiring.
Merge pull request #463 from LudwigOrtmann/native_hwtimer_fixup

fix native hwtimer