- 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!

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?

- Fix bug in ubl_line_to_destination
- Improve/fix output of some ubl.h functions

add BLTouch-related messages in english and (rusty) french;
add missing endstops.h in ultralcd.cpp;
fix misc. compiler warnings;
fix lsf_reset - ZERO macro can't handle a pointer as it would only memset the size of the pointer, not the size of the entire struct

Also...   The memory corruption issue may be fixed.   The GCC compiler
was inlining static functions and this caused the G29() stack frame to
become much larger than the AVR could handle.

* relocated ubl state to config. store:
 * removed a number of ubl state variables and padding which were largely unused - saved 58 bytes of both SRAM and EEPROM;
 * modified ubl sanity_check - no longer checks removed state variables that were otherwise unused, where checking didn't seem to accomplish anything, ultimately;
 * removed pre_initialized state, saving 64 bytes of SRAM;
 * removed automatic saving of UBL state after UBL activation/deactivation;
* consolidated multiple GRID_MAX_POINTS_X/Y to 'Global Leveling' section of EEPROM;
* minor update to G29 Sx notes/instructions;
* renamed mesh load and save parameter to 'slot' from 'm' for clarity;

fix maximum mesh_index_ array size at 16 (15+1);

- Make all `unified_bed_leveling` data/methods static
- Move some UBL-related variables into the class
- Replace `map_[xy]_index_to_bed_location` with `mesh_index_to_[xy]pos`

UBL's maps are cleaner.   And with a 1 value, G29 O 1  will generate a
map suitable to be Cut & Pasted into Excel so a Suface Map can be
generated.