nat: Move MSC ip address into the config..

The address can still be specified on the cli and it will
overwrite the config in the config file.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 1583306..623bec5 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -51,9 +51,17 @@
 	nat->stats.bsc.reconn = counter_alloc("nat.bsc.conn");
 	nat->stats.bsc.auth_fail = counter_alloc("nat.bsc.auth_fail");
 	nat->stats.msc.reconn = counter_alloc("nat.msc.conn");
+	nat->msc_ip = talloc_strdup(nat, "127.0.0.1");
 	return nat;
 }
 
+void bsc_nat_set_msc_ip(struct bsc_nat *nat, const char *ip)
+{
+	if (nat->msc_ip)
+		talloc_free(nat->msc_ip);
+	nat->msc_ip = talloc_strdup(nat, ip);
+}
+
 struct bsc_connection *bsc_connection_alloc(struct bsc_nat *nat)
 {
 	struct bsc_connection *con = talloc_zero(nat, struct bsc_connection);