Support new Pi models
- Raspberry Pi 400: verified as booting to desktop.
- Compute Module 3+: some changes to SD support to make it behave like the
plain Compute Module 3 (previously it was falling into the model B+ code
path and setting up GPIO in the expectation that an activity LED was
attached, which is not a given for a Compute Module). Not tested.
- Compute Module 4: tentative support added. Not tested. In particular, we
don't know what the revision numbers will be yet, so the entries in
GPIO_Board_Conversion_Table may not match real hardware.
- SD subsystem now assumes any future models are similar to the Pi 4 and 400,
and thus we're more likely that they will "just work" out of the box.
