systemd: add network unit file

Add and enable a systemd unit file to bring up or down network with ifup /
ifdown, analogous to the skeleton/etc/init.d/S40network init script.

Signed-off-by: Ivan Sergeev <vsergeev@kumunetworks.com>
[eric.le.bihan.dev@free.fr:
  - rebase
  - install service only if systemd-networkd is not selected]
Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>