Add loading of config data from node_config.json (LittleFS)

[matthias-bs]

Intended for configuration data which is hardware (e.g. power supply) or deployment environment (e.g. network, timezone) dependent and not expected to change after deployment.

node_config.json example:

{
    "timezone": "CET-1CEST,M3.5.0,M10.5.0/3",
    "battery_weak": 3310,
    "battery_low": 3410,
    "battery_discharge_lim": 3210,
    "battery_charge_lim": 4190,
    "battery_capacity": 2200
}

If present, the value from the file overrides the default from the source code.

Implementation similar to secrets.json.