gf-network-conf: Add generate volatile networkd config files during boot

Did some reorganization in the code to reuse more code. Add parameter to generate config files, default is just to print them to stdout. Add additional output format for networkd. Detect if networkd or networkmanager config format is needed. Allow to specify format on the commandline

As reading a file from etc shared has the same impact on boottime as using gf-network-conf during boot to write the config files directly into the tmpfs each time, this seems to be the simpler solution to get the network configs into the network utils. Networkd is enabled for fngsystem and started after shared mount and volatile files generation.