systemd: Remove networkd depending on gplv3 in DISTRO_FEATURE, configure MAC Address

Normally we would use network-manager, but for the gplv3 free variants
it is not available. In these cases we keep systemd-networkd as
network tools available.
Also adapt the call to gf-network-conf in the postinsts step during the
first boot to automatically determine the used network tooling.
This way the configuration for systemd-networkd is generated from the
xml setting as needed.

Until now the MACAdress was not configured if systemd-networkd was used.
Now the MACAddress is added to the newwork config file like it is done
when network-manager is used.

(cherry picked from commit 91b1c14e)