- Also fix handling of SCARA and misuse of `codenum`.

See: #6777

I believe gcode_M605 will not be called when using DUAL_NOZZLE_DUPLICATION_MODE without this change.

Changed UBL_G26_MESH_EDITING to UBL_G26_MESH_VALIDATION
Added UBL_MESH_EDIT_MOVES_Z to prepare for that change (coming next)

Reference: #6804

- can be augmented in the future to enable for other leveling systems

Quite simple, but did not want to modify `G1` as the additional checking would slow it down.

Tested & working.

- fix broken `M421` due to less-than-careful optimization
- add HOME_AFTER_DEACTIVATE define to advanced config so not everyone has to rehome after steppers are deactivated
- misc. cleanups (remove unused label, unused variables)

 - Add crc16 utility function
 - Implement CRC16 for config store, remove old checksum, increment layout version
 - Move UBL mesh store/load to MarlinSettings; increment UBL_VERSION
 - Begin to lay out MAT structure, prototype functions, etc.
 - Rename ubl.state.eeprom_storage_slot to .storage_slot
 - Misc. optimization
 - Cleanup/standardize/improve some messages

This is a work in progress!

Also clean up ubl_motion.cpp debug info and fix declaration of cx & cy

Extra closing curly bracket terminates while loop early if PRINTER_EVENT_LEDS defined

UBL on Delta's....     Should be close!    Should not affect any Cartesian printer.

- Split M421 into separate versions for bilinear and ubl
- Fix minor issue in G26

* M421 Mesh Point Offset and misc. UBL clean up

Allow M421 to accept an offset as well as absolute value for a specified
mesh point.
And misc. UBL clean up to reduce redundent code.

* Better error checking for M421

* Fix M421 Y index bug

I just noticed....  We've had a Y index bug for who knows how long?

Saves a few lines of code.