Stop using fake EEPROM, store settings as JSON in file system

[henrygab]

Having two methods to read/parse/store configuration (EEPROM and JSON) is error-prone (de-synchronized), and increases maintenance costs. See discussion in #216.

Thus, it makes sense to store (at least) user-configurable data in /config.json, and to stop using the (fake) EEPROM ... especially as that only writes to a single sector (burn out the flash sector). LittleFS has more overhead, but as reading / updating the configuration is not a performance-sensitive operation, the benefits in maintainability will likely outweigh the downsides.