Figure out how to access sysfs gpio without requiring privileged flag

[shaunmulligan]

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"