gprs_ns2_sns: Allow VTY configuration of default binds for IP-SNS

In the IP-SNS SGSN role, we need to inform the BSS of our local
IP endpoints.  For statically configured NSEs, those are explicitly
stated on a per-NSE level.

For dynamically created IP-SNS NSEs, we are adding a new VTY
command, using which the administrator can configure which binds
should be advertised as IP endpoints to such BSS.

Change-Id: Id01c29b07e9203c9305f2129361a4f5aaefa2c52
Related: OS#3373
diff --git a/src/gb/gprs_ns2_internal.h b/src/gb/gprs_ns2_internal.h
index 567e1c5..29717c6 100644
--- a/src/gb/gprs_ns2_internal.h
+++ b/src/gb/gprs_ns2_internal.h
@@ -388,3 +388,6 @@
 enum gprs_ns2_vc_mode ns2_dialect_to_vc_mode(enum gprs_ns2_dialect dialect);
 int ns2_count_transfer_cap(struct gprs_ns2_nse *nse,
 			   uint16_t bvci);
+
+/* vty */
+int ns2_sns_add_sns_default_binds(struct gprs_ns2_nse *nse);