[Feature] Digipots support

[johnboiles]

I've got a change in my branch to support Digipots for setting current on stepper drivers digitally. I added this to support my Ultimachine RAMBo 1.4 board. Is this something we'd want to include in the main grbl repo? I'm happy to make a PR to merge it in if so. Please advise!

[chamnit]

Thanks! I think I'd be open to adding this to the Grbl Mega branch, as there is plenty of room to add this there.

[johnboiles]

@chamnit great! So is the idea to keep this repo as minimal as possible and then add other features to processor-specific forks such as grbl-mega? Should I do the same for #695? I’ll of course keep everything behind #defines so that it can be compiled out for people who don’t need it

[johnboiles]

@chamnit I put up my pull-request here: gnea/grbl-Mega#108

One thing for us to consider, is that if we're adding a per-axis setting to gnea/grbl-Mega and not gnea/grbl, we'll probably want to at least reserve those settings codes in gnea/grbl so that we don't diverge on which setting codes do what.

Alternately, we could probably put this into gnea/grbl, since it's fully compiled out if HAS_DIGIPOTS is not defined.

Take a look at the PR and let me know what you think!