Add custom Tim7Delay

[dbrgn]

The HAL delay uses the systick timer (SYST). This timer is not available anymore when using RTIC with the scheduler.

Here's an implementation of DelayUs and DelayMs for TIM7 (with busy-looping until the update event of the timer is reached). It seems to work, although delays lower than ~4 µs cannot be achieved.

The precision seems to be good enough for one-wire to work:

Busy-looping was the easiest approach and works, but seems a bit wasteful. Is this busy-looping standard practice? Would it be better to schedule an interrupt and WFI during looping? (I'm not sure if this could be implemented cleanly in a standalone implementation because interrupt handling ist done by RTIC...)

I think in our case it should be fine because we only use the blocking delay for very short intervals during things like one wire communication.

[rnestler]

Busy-looping was the easiest approach and works, but seems a bit wasteful. Is this busy-looping standard practice?

For short delays (<1ms) there is often no other way than busy-looping, since going to sleep and waking up again would take too long.

[dbrgn]

:+1: Do you want to review the code in-depth, or should we merge as-is?

[dbrgn]

@rnestler ping :slightly_smiling_face:

[dbrgn]

Example updated, fixup commits squashed.