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.

PUBLISHED_FROM=f7d582421a8d7e4d1ed50a280f2670d8b62f8d45

Author: dimonomid