Fixes#795
Replace all CoreMutex and Newlib mutex accesses with FreeRTOS calls
when in FreeRTOS mode. Avoid issues with hange/etc. due to priority
inversion.
No changes to normal operating mode.
Add a FreeRTOS stress test that caught the issue fixed here.
Using all the work from @hfellner and others, add FreeRTOS
SMP support.
Allow idling cores through the FreeRTOS FIFO queue to
allow for file system and EEPROM support.
Make delay a weak function so FreeRTOS can override.
Add cycle count support under FreeRTOS using a PIO SM.
Use a task-based approach for handling the USB periodic work
instead of the IRQ-based one in the main core.
Set 8 prio levels so it fits in 3 bits nicely (0..7).
