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FreeRTOS SMP was updated to:
a) Move ths SYSTICK handler, which cannot be disabled and can fire
   even with IRQs disabled, to RAM
b) Add a flag from the core to the SYSTICK handler to hold off on
   any PendSV (task switch) calls while we are doing the idleOtherCore.

The core now sets this flag, _holdPendSV, and adds add'l FreeRTOS SMP
calls to really, really tell the OS we can't, don't, and better not
be swapped out while writing to flash.

Fixes #719