Hardware target reset support

[newbrain]

A small PR to introduce support for a hardware target reset.
Asserting and de-asserting the reset signal is controlled by a new probe USB command:
https://github.com/newbrain/picoprobe/blob/5f1630c390fbb912b625231d6c02fb7b3b538b27/src/probe.c#L76

If the value passed in the bits command header is false (0) the reset line is de-asserted (high).
If true (anything else) the reset line is asserted (low).

The reset pin is configured in picoprobe_config.h, currently GPIO 6:
https://github.com/newbrain/picoprobe/blob/5f1630c390fbb912b625231d6c02fb7b3b538b27/src/picoprobe_config.h#L55-L56
Open drain is simulated changing the pin direction from output (for low) to input (for high, with pull-up)

This can be easily added to OpenOCD, and I'll include it in pyOCD when I propose the picoprobe support PR.

Tested with a modified pyOCD and a scope.

[newbrain]

@liamfraser:
Just to make sure, is this project active?
(I know that pesky thing called "life" has its ways to interfere)

The Picoprobe is quite handy if you have no other probes, and since the Pico boards are selling like hot cakes, I can imagine there is a good number of persons in that situation that might want to try the thrill of a real debugging session.

I've now added support in pyOCD (PR still open: pyocd/pyOCD#1083 some improvements needed).

[liamfraser]

Hi - yes. Sorry, I missed this. Been busy working on other stuff. Looks good to me so will squash and merge.

[newbrain]

Thanks!