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/bsc/osmo_bsc_main.c b/openbsc/src/bsc/osmo_bsc_main.c
index 7ddf91d..e84450f 100644
--- a/openbsc/src/bsc/osmo_bsc_main.c
+++ b/openbsc/src/bsc/osmo_bsc_main.c
@@ -22,8 +22,8 @@
 #include <openbsc/debug.h>
 #include <openbsc/gsm_data.h>
 #include <openbsc/osmo_bsc_rf.h>
+#include <openbsc/vty.h>
 
-#include <osmocom/vty/command.h>
 #include <osmocore/talloc.h>
 #include <osmocore/process.h>
 
@@ -137,6 +137,7 @@
 	.name 		= "OpenBSC Osmo BSC",
 	.version	= PACKAGE_VERSION,
 	.go_parent_cb	= bsc_vty_go_parent,
+	.is_config_node	= bsc_vty_is_config_node,
 };