Revamped homing cycle. Axis limits and max travel bug fixes. Build info. Refactored config.h.

- Revamped and improved homing cycle. Now tied directly into the main
planner and stepper code, which enables much faster homing seek rates.
Also dropped the compiled flash size by almost 1KB, meaning 1KB more
for other features.

- Refactored config.h. Removed obsolete defines and configuration
options. Moved lots of “advanced” options into the advanced area of the
file.

- Updated defaults.h with the new homing cycle. Also updated the
Sherline 5400 defaults and added the ShapeOko2 defaults per user
submissions.

- Fixed a bug where the individual axes limits on velocity and
acceleration were not working correctly. Caused by abs() returning a
int, rather than a float. Corrected with fabs(). Duh.

- Added build version/date to the Grbl welcome message to help indicate
which version a user is operating on.

- Max travel settings were not being defaulted into the settings EEPROM
correctly. Fixed.

- To stop a single axis during a multi-axes homing move, the stepper
algorithm now has a simple axis lock mask which inhibits the desired
axes from moving. Meaning, if one of the limit switches engages before
the other, we stop that one axes and keep moving the other.