Updated G28/G30 intermediate motion behavior.

- G28 and G30’s behavior has been updated from the old NIST g-code
standard to LinuxCNC’s. Previously when an intermediate motion was
programmed, the NIST standard would move all axes to the final G28/30
stored coordinates. LinuxCNC states it only moves the axes specified in
the command.

  For example, suppose G28’s stored position is (x,y,z) = (1,2,3) for
simplicity, and we want to do an automated z-axis tool retraction and
then park at the x,y location. `G28 G91 Z5` will first move the Z axis
5mm(or inches) up, then move Z to position 3 in machine coordinates.
Next, the command `G28 G91 X0 Y0` would skip the intermediate move
since distance is zero, but then move only the x and y axes to machine
coordinates 1 and 2, respectively. The z-axis wouldn’t move in this
case, since it wasn’t specified.

This change is intended to make Grbl more LinuxCNC compatible while
making commands, like the shown tool retraction, much easier to
implement.

README.md

@@ -38,6 +38,8 @@ Grbl includes full acceleration management with look ahead. That means the contr
 
 - New '$' Grbl settings for max and min spindle rpm. Allows for tweaking the PWM output to more closely match true spindle rpm. When max rpm is set to zero or less than min rpm, the PWM pin D11 will act like a simple enable on/off output.
 
+- Updated G28 and G30 behavior from NIST to LinuxCNC g-code description. In short, if a intermediate motion is specified, only the axes specified will move to the stored coordinates, not all axes as before.
+
 - A few bug fixes and lots of refactoring to make the code more efficient and flexible.