Add missing thread yield system call.

Add missing thread yield system call.

This change fixes a bug where the threads were not treated
fairly.  Before the fix, empth_yield would only yield to
threads already waiting hThreadMutex mutex.  It would not
yield to other threads ready to run from the release of other
mutexes.  An example of this is that the update task did
not start when force command was issued from script sequence.