Non-volatile memory data repository pattern

[tzarc]

Description

Start of refactoring for non-volatile storage other than EEPROM. Converts eeconfig to be a shim, offloading to an nvm_eeconfig equivalent which in turn does the EEPROM writes as of today. Same transformations done for via/dynamic_keymap.

New storage mechanisms like the incoming filesystem APIs can then implement nvm_eeconfig in its own terms, which may mean one file per concern, as an example. It also paves the way for providing read/write capability for other subsystems, such as combos, key overrides, etc. -- unlikely to be implemented in the EEPROM-backed NVM driver due to a lack of space, but may be available for others.

Tested on Satisfaction75 with VIA and its custom configuration stuff, seems to work fine.

TODO:

• eeprom debounce helper stuff

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).