core: make sched_runqueue public

Reverting b604832, because thread_yield() needs to access this
information.
core: move thread_yield*() to thread.[ch]

Although it might conceptionally rather belong to the scheduler, the
yield functions are prefixed with thread_ and thus, belong there.
Merge pull request #1885 from OlegHahm/thread_yield_higher_cleanup

core: thread_yield_higher cleanup