Idea: "barebones" Duet3D mainboard, maybe as Pi hat

[T3P3Tony]

@sonderzug et al

We have been considering a SAME70 CAN-FD control board designed to use CAN-FD expansion for most functions for a long time now, but need to consider the total stock of these processors available to use before adding another product that uses them.

If we did it Pi hat is one form factor that could be used but may not be optimal.