Figure out how to access sysfs gpio without requiring privileged flag

Currently to use the balena fin boards RGB led, we need access to /sys and the only way I have been able to do that so far is by enabling `privileged: true` for the service.

It would be nice to figure how to avoid using privilege and map the serial devices in as such:
```
    devices:
       - "/dev/UC20.NMEA:/dev/UC20.NMEA"
       - "/dev/UC20.AT:/dev/UC20.AT"
```