NS: Add support for persistent NS-VC configuration

With persistent NS-VC configuration (configured through VTY),
we can respond properly to BSS with a somewhat strange NS
implementation Such as the BSplus.  It enables us to respond
with a proper NS-RESET (including NSVCI/NSEI) when receiving
a NS-ALIVE or other PDU for a BLOCKED/DEAD NS-VC after our
end of the connection is rebooted.
diff --git a/openbsc/src/vty/command.c b/openbsc/src/vty/command.c
index 67e6804..fe29824 100644
--- a/openbsc/src/vty/command.c
+++ b/openbsc/src/vty/command.c
@@ -2365,6 +2365,7 @@
 	case MGCP_NODE:
 	case GBPROXY_NODE:
 	case SGSN_NODE:
+	case NS_NODE:
 		vty->node = CONFIG_NODE;
 		vty->index = NULL;
 	default: