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.
Earle F. Philhower, III
01ee673dc2