New form factor

@neilg, your demo with 4 motors was impressive so I'm keeping an eye on this project! I see you now have a combined board that mounts on the stepper's back, looking good. I actually started designing the same thing in KiCad yesterday, it's virtually identical but with Jake's ucbus stepper format so it can mount on either NEMA-14 or 17:

I might publish it under 865.21/projects, I don't want it to be redundant though.

Two small deltas:

• M0, M1 and SLEEP go to the SAMD to have software-control
• 2.54mm JTAG pins: you can connect an endstop switch for free on e.g. the SWDIO and GND pins