[PR 753 rework][SoftwareSerial] Add function to set interrupt priority

[fpistm]

This PR is a rework of #753.
Instead of allow to be able to change the interrupt priority of the Timer used by the SoftwareSerial library, it allows to change interrupt priority of all the timer used through the HardwareTimer class.

Fixes #751

[sjasonsmith]

I like making this available for all hardware timers, instead of my change for only SoftwareSerial.

I think this will also solve another issue, which is the need to re-set the priority every time the timer is resumed.

I would like to test this on hardware, but will not have time to do so until tonight. You should have my results in the morning!

[fpistm]

@sjasonsmith
Thanks

About:

I think this will also solve another issue, which is the need to re-set the priority every time the timer is resumed.

Well this is the case even with this PR because it is done by the HAL_TIM_Base_MspDeInit which is called by HAL_TIM_Base_Init each time resume is called.
Anyway, compared to the first implementation you've made, this is done only once instead of twice in setSpeed (one by the resume then call to NVIC_SetPriority)

[sjasonsmith]

@fpistm

I reworked this into my Marlin branch, and confirmed that it is working.

I am able to rearrange my three timers (2 HardwareTimer + 1 SoftwareSerial) in any order and verify proper preempting behavior.

I also verified that I can now set the priority once when initializing the HardwareTimer, rather than after resume as was previously required.

[fpistm]

@sjasonsmith
Thanks for tests.
In that case I will merge this one instead of #753.
I will wait @ABOSTM feedback as he is the author of the HardwareTimer library.

[LinoBarreca]

@fpistm I highly approve this PR (although it breaks interface compatibility with Arduino's HardwareTimer) it's kinda necessary on STM32 platform.

[ABOSTM]

Looks good to me.