Integrate Holey and Standard firmware versions

[emilecantin]

This issue's aim is to discuss and plan what needs to be done in order to bring back the Holey calibration / kinematic model code back into the main branch.

[krkeegan]

Slowly jumping back into things.

I would highly suggest using a $ setting for the switch between calibration types. In the beginning we used GCode and it is a dangerous proposition (1) we may commandeer a command that might be later enabled on the Maslow; or (2) more importantly, a GCode generator may spit out on of those commands and could create havoc on a user's machine.

[Orob-Maslow]

The difference is just in the kinematics... I just submitted a pr for supporting a different shield and a few other things, but this should be the next fold-in so a flash is not required to switch from triangular to holey calibration. Any suggestions on what $ number should be used? The z-axis setpoints are stored using B commands. I think they should move to $ commands as well, though really a separate discussion could be had for that.