* Get UBL Mesh Generation, Mesh Save & Mesh Load working with 32-Bit platforms

* clean up read_data() and write_data() for non-LPC1768 HAL's

* Get read_data() and write_data() return codes consistent

All HAL's read_data() and write_data() return false if they succeed.

* Get read_data() and write_data() return codes to be consistent

Make read_data() and write_data() return true if an error happens.

* Say UBL is now checked out on machine types in default Configuration.h file.

Implement AVR Platform

- 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!

* 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?