Use USB-C debug accessory mode for programming firmware

[mriise]

USB-C alternate mode detection, could be useful to turn the USB port into a debug accessory for debugging work thru just a USB-C cable and an adapter chip.

There are a host of considerations to make before this will be a real option. such as

• USB IF certification requires that entering into debug accessory mode is extremely intentional and known by the user.
• this will only detect change in modes, not route the pins. there may be another chip that would do this
• using the 2 extra pins can get SCLK and SWDIO for flashing, what other 2 pins can be re-used for RESET and SWO? (probably steal one of the D+/- pairs)

[mriise]

1: can be addressed by requiring that all firmware changes are authenticated (which requires secure boot) 2: this is easy enough to check on boot for the main mcu, switch the pins, then self-reset. 3: added requirement that this should be reverseable.

making a debug probe and no. 1 is probably the biggest burden here. PTN5150H is able to detect debug acessory, so we are all good on that front.

atm i think putting an rp2040 as the probe using modified picoprobe firmware. Though all of this should be fairly down the line in scope. using a normal debug probe setup will work fine for a good while. https://github.com/raspberrypi/picoprobe