Factory config mode should wipe obsolete prefs from NV storage

[lars-t-hansen]

Occasionally prefs keys are obsoleted (though never reused, se config.cpp). When we upload a new factory config we should unconditionally remove or set to default the values for all obsoleted keys so that secrets don't leak from old configs left on devices.

The problem currently pertains only to a few names: loc, tsh, tsp, huh and hup, none of which are particularly security-sensitive. So this is not an urgent problem, just a hygiene thing.