Add user programmable button

[ajn96]

Add button which can be reconfigured on the fly by software. A BTN_CONFIG register could be created, which maps the button press to a bit in the command register. This would allow users to start/stop SD scripts, reset DUT, flush buffer, etc while operating headless.

Ideally it would be nice for button to be rotated 90 degrees relative to the board (accessible from the side), but in plane with the micro could also work (primary use case is for headless data logging).

[ajn96]

@juchong I will figure out what pin would be best for this - ideally a GPIO which doesn't share an interrupt with other parts of firmware

[ajn96]

This has been incorporated into the rev C board design