We only execute one thread at a time, and we do this by using a mytex that
the executing thread locks. sleep() returns the mutex, sleep and then gets
it again, yeild() makes the same as sleep but immediately.
This makes threading in the simulator behave closer to how it works on target
and it makes it necessary to use yield() or sleep() for other threads to
get to execute.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@1002 a1c6a512-1295-4272-9138-f99709370657
