package/network-manager: Enable service under systemd

Previously NetworkManager had to be enabled and started on the
first boot manually or by a script.

Add install define with the commands to setup the required
service files for automatic start.

Signed-off-by: Nathaniel Roach <nroach44@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>