vty: Provide a is_config_node for the VTY and use it for the bsc

Right now only bsc_hack and osmo_bsc need to have a custom
config handling as they use the subscr/oml nodes.
diff --git a/openbsc/src/common_vty.c b/openbsc/src/common_vty.c
index 3c257a8..9d22146 100644
--- a/openbsc/src/common_vty.c
+++ b/openbsc/src/common_vty.c
@@ -185,3 +185,15 @@
 	return CMD_SUCCESS;
 }
 
+int bsc_vty_is_config_node(struct vty *vty, int node)
+{
+	switch (node) {
+	/* add items that are not config */
+	case OML_NODE:
+	case SUBSCR_NODE:
+		return 0;
+
+	default:
+		return 1;
+	}
+}