vty: CONFIG_NODE is the anchor and should not be treated as config.

This is fixing an assert in vty_go_parent due trying to go from
CONFIG_NODE to the parent.
diff --git a/openbsc/src/common_vty.c b/openbsc/src/common_vty.c
index 9d22146..b2a394f 100644
--- a/openbsc/src/common_vty.c
+++ b/openbsc/src/common_vty.c
@@ -191,6 +191,7 @@
 	/* add items that are not config */
 	case OML_NODE:
 	case SUBSCR_NODE:
+	case CONFIG_NODE:
 		return 0;
 
 	default: