systemd-conf: do not ask for DHCP if configured on kernel command line

In cases where we configure the IP address and more on the kernel
command line with ip= we should not ask for DHCP with systemd-networkd
later on. We have such a setup with our runqemu script.

With this match in place we can also deploy this unit on qemu systems.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8c4981e7)