Write and retrieve configuration from persitent memory

[krzmaz]

Useful resource: https://www.makermatrix.com/blog/read-and-write-data-with-the-pi-pico-onboard-flash/

Due to the wear considerations (flash having the lifetime of 1000-10000 write-erase cycles), best would be to start with https://github.com/littlefs-project/littlefs as CMake external project dependency. If that's too problematic, a simpler approach can be considered with proper warnings to the users, see: https://arduino-pico.readthedocs.io/en/latest/eeprom.html.

Within this issue, retrieving the configuration during preparation of the configuration page and writing the data entered in the configuration page to flash is enough.