nfs: Add ip=... and root=nfs... parameters to internal dracut cmdline

If the rootfs is an nfs mount, also know as nfsroot, add the correct
parameter to the dracut cmdline.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Fix error message when there are no internal kernel commandline parameters

Otherwise you could get messages like that:
Stored kernel commandline:
/usr/bin/dracut: line 1559: /var/tmp/initramfs.psHn4a/etc/cmdline.d/*.conf: No such file or directory

Signed-off-by: Thomas Renninger <trenn@suse.de>