Some improvements to optionrom preemption support.

Some improvements to optionrom preemption support.

Enable preemption during VGA mode switch call - this call can take
several milliseconds on real hardware.

Call yield() in finish_preempt() - when preemption is configured it
allows threads in wait_preempt() to run; when not configured it gives
an opportunity for threads to execute after the implicit delay from
optionrom execution.

Don't penalize priority in run_thread().  The run_thread() code would
implicitly yield because it created the new thread on the list after
the current thread and then jumped to it.  When in a preemption event,
a yield effectively waits approximately one millisecond (to the next
rtc irq).  The implicit yielding in run_thread thus limited the number
of threads one could launch during preemption to 1 per millisecond.
So, change the code so that the new thread is created prior to the
current thread - thus eliminating the effective yield from
run_thread().