Introduced a configuration for uninterruptible task source event.

Now, by default, HRTIM will be the source of the uninterruptible task trigger.
However, as this require HRTIM to be initialized, Timer 6 is kept as an option for uses that do not involve HRTIM.

Also:
- Introduced a function to obtain HRTIM period in µs, and corrected a misleading comment on the existing function.