Implement AVR Platform

 - 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!

Following up on #3231

Apply `ENABLED` / `DISABLED` macros to EEPROM-related files.

- Apply standards to cardreader.*
- Fix minor issues with cardreader.cpp
- Apply standards to some other stale regions

Update ConfigurationStore to always write dummy values for disabled
options, including FWRETRACT, DELTA, and SCARA. Update the EEPROM
version to “V15.” Also fixes a buffer overrun with axis_scaling in
Config_ResetDefault.

Changed EEPROM CHITCHAT behavior. M503 is always enabled.