Don't use RTC to time boot menu delay.

Don't use RTC to time boot menu delay.

It appears real machines sometimes have a flaky RTC, so avoid using
the RTC irq during boot.  Instead, use a delay based on the standard
timer irq.

This also optimizes CONFIG_THREAD_OPTIONROMS users as it is no longer
necessary to use preemption - the wait_irq() call handles task
switching natively.