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Fixes #1394

The Pico_Rand SDK calls gather bits from the HW ROSC at precise intervals.
If there is jitter in the sleep_until() call then the ROSC bit collection
will always think it's failed to acquire the right bit and retry infintitely.

Avoid by wrapping the HW random number calls and the sleep_until() routine.
Only when in FreeRTOS set a flag to silently make sleep_until() into a
busy wait loop while in a random number generation step.  When not in the
random code, do the normal sleep_until call.