network-manager: generate configuration with netroot=

If the root is on network, let nm-initrd-generator create configuration
even if none was explicitly specified on the command line.

Also do the same if /tmp/net.ifaces exists, because the anaconda plugin
creates an empty file in that location in hopes that will make us
configure the network.

modules.d/35network-manager/nm-config.sh

@@ -1,3 +1,7 @@
 #!/bin/sh
 
+if [ -n "$netroot" ] || [ -e /tmp/net.ifaces ]; then
+    echo rd.neednet >> /etc/cmdline.d/35-neednet.conf
+fi
+
 /usr/libexec/nm-initrd-generator -- $(getcmdline)