[VTY] Introduce "end" command that works from any level in config

Using "end" you can always return to the "enable" level, and from
there the "show" commands are available.  So no more need for
exit/exit/exit/exit/disable.
diff --git a/openbsc/include/openbsc/vty.h b/openbsc/include/openbsc/vty.h
index c6fc203..1b67589 100644
--- a/openbsc/include/openbsc/vty.h
+++ b/openbsc/include/openbsc/vty.h
@@ -15,5 +15,6 @@
 extern struct cmd_element cfg_description_cmd;
 extern struct cmd_element cfg_no_description_cmd;
 extern struct cmd_element ournode_exit_cmd;
+extern struct cmd_element ournode_end_cmd;
 
 #endif