core: sched_switch only switch for higher priority

sched_switch() is called by some library functions when a call unblocks
another thread. Then it needs to be tested if the current thread should
be preempted for the newly runnable thread.

A non-volutarily yield should only happen if the unblocked thread has a
_higher_ priority than the current thread. The current implementation,
which tests if the other thread has the same or a higher priority, does
not fit the documentation.
Merge pull request #1693 from Kijewski/sched_switch-only-if-higher

core: sched_switch only switch for higher priority