Omit combustion module in initrds for already configured systems

If the initrd is hostonly and /etc/machine-id already exists, don't include
the combustion dracut module and its dependencies.

README.md

@@ -173,6 +173,9 @@ subsequent boots.
 
 ### Combustion
 
+The `combustion` dracut module is included by default, but omitted if dracut is
+run on an already configured system in `hostonly` mode.
+
 `firstboot.target` pulls in `combustion.service` and
 `combustion-prepare.service`. The latter runs after the config drive or
 QEMU fw_cfg blob appears (see `combustion.rules` for details). The combustion

module-setup.sh

@@ -1,3 +1,11 @@
+check() {
+	# Omit if building for this already configured system
+	if [[ $hostonly ]] && [ -e /etc/machine-id ] && ! [ -e /var/lib/YaST2/reconfig_system ]; then
+		return 255
+	fi
+	return 0
+}
+
 depends() {
 	echo bash firstboot network systemd url-lib
 }