fix(network-manager): only run NetworkManager if rd.neednet=1

fix(network-manager): only run NetworkManager if rd.neednet=1

Don't run the new systemd unit (nm-run.service) if rd.neednet=1
isn't set. nm-initrd-generator will generate configuration even
without rd.neednet=1 so determining if we should start based on
just if connection profiles exist isn't enough. We need some other
indicator. In this case we lay down a /run/NetworkManager/initrd/neednet
if rd.neednet=1, which is used by nm-run.service to determine the
need to run.