Add mechanism to set SoftwareSerial interrupt priority

[sjasonsmith]

Summary

Add a mechanism to set timer interrupt priority for the timer used By SoftwareSerial.

This PR fixes/implements the following bugs/features

• Allow override for SoftwareSerial timer ISR priority #751 Allow override for SoftwareSerial timer ISR priority

Validation

I have reworked an in-development version of Marlin to use this to configure the SoftwareSerial interrupt priority during HAL initialization. Using digital outputs added to the interrupt handlers, I was able to confirm the nesting of multiple preempted timer interrupts, as desired.

In the image below it can be seen that the serial interrupt is the highest of the three priorities, allowing it to preempt both of the other timer interrupts.

Fixes #751

[fpistm]

Hi @sjasonsmith
Thanks for this PR, during the review, I've though this will be more generic to add a mechanism to set timer interrupt priority for the timer used By HardwareTimer then it is possible to do it for SoftwareSerial.
Could you check #755 and give us your feedback.

[fpistm]

Replaced by #755