CF4 feature set and requirements

[pavel-kirienko]

• Two CAN FD interfaces, each managed by a separate displayless CF3.

• An updated BugFace based on a larger chip that can run the stock Black Magic Probe firmware without modification (with all targets supported).

• USB Type C connector.

• Instead of a USB host connector, add a separate RP2354 for general-purpose control features, connected to the same USB hub. This is a must for any nontrivial mass production fixture. This one could use a small 7-segment indicator or a cheap display.

• Power supply output does not work stably so it needs to be removed, unless we were to add USB PD support (which we probably don't want).

• Full galvanic isolation from the host is nice to have (e.g., LTM2884 -- fully integrated single-chip data/power isolator).