Store details of the EEPROM layout in both flash and EEPROM so that we can be smarter about reinitialization of EEPROM after an incompatible firmware update

[obra]

Right now, flashing a firmware update that changes the eeprom layout onto a device results in scrambled keymaps. That's...less than ideal. It'd be better to wipe the EEPROM (or entirely disregard it) in cases like that.

Longer term, on devices with more storage, we should move to a configuration system that is not based on exact indexes into the EEPROM.