Adjust stepping and CPU map to eschew rewiring.

[tlhonmey]

Very simple really, GRBL does its step timing calculations and scheduling on one set of bitmasks,
therefore in the original setup, all GPIO pins for each operation must be on the same port.

However, it is simple enough to create the bitmask as 32-bits wide, let GRBL do all its math,
and then split it in half and write it to two separate ports as necessary.

I may expand this approach to additional functions as I continue to suss out which pins are attached to what.

[tlhonmey]

Note that you don't have a branch to separate out the USBMach from whatever custom board you're moving on toward, and I don't know how to tell this silly thing to merge further back.

I don't know if you care about this board any more, but they seem plentiful and cheap online, so having software for them that actually works is nice.

Can you tell me how to make the spindle control on this actually work? I can't manage to sort out what pins it's attached to.