BREAKING: Sys config API change

Now apps should use getters and setters instead of accessing struct
fields directly, e.g. instead of `get_cfg()->update.timeout` it should
be `mgos_sys_config_get_update_timeout()` to get the current value, and
`mgos_sys_config_set_update_timeout(123)` to update the value.

For now, the config structs are public, but they will be made private
soon, so use accessors to keep your code working.

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