Based on Infitary's own firmware settings.

===============================================

also an issue with ubl_motion.cpp

Also corrected compile issue when DEBUG_GCODE_PARSER and
AUTO_BED_LEVELING_UBL are both enabled.

Allows the user to send text to the serial console in order to
communicate with a host - sending debuging information or action
commands, for example.  Text must begin with '//' and this is added if
it is not already present at the beginning of the string.

Followup to #6940

Can't compile with PINS_DEBUGING enabled on Atmega1284p because it tries
to enumerate pins 46 and 47 (which don't exist) because `#ifdef
AVR_AT90USB1286_FAMILY ` doesn't work as expected.

Fixup length

Fixes #7150

- Added `H` to set a nozzle height adjustment to surface of the bed.
- Updated G29 P4 comments to reflect changes.

Addressing #6992 and #5851