Use link-local addresses for USB gadget NICs

Add systemd-networkd, since it provided the easiest solution to get
link-local addresses directly (instead of only after DHCP discovery
times out).

Task-number: QTBUG-55434
Change-Id: I4198514a0bfef9d65da33f27af64a5055567dd9b
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>