Feature: Reserved priorities

[korken89]

When an MCU has a softdevice (e.g. for radio) it's common that they reserve some priorities that the system is not allowed to use. We should add a way to exclude prios.

[TheOpenDevProject]

This would drag the project dangerously into the realm of being MCU specific unless they HAL provided a way to notify rtic of this

[AfoHT]

The developer indicating which priorities to avoid should not make this MCU specific I don't think.

If the BSPs/HALs could provide this list that could be fed into RTIC even better, and we should strive to have that compatible with the user manually specifying which to avoid.