Add VTY option to set GPRS network-control-order to enable MS measurements

In order to enable GPRS downlink measurements at mobile, the
network-control-order must be set to nc1.
diff --git a/openbsc/src/libbsc/system_information.c b/openbsc/src/libbsc/system_information.c
index acf7134..88b81dc 100644
--- a/openbsc/src/libbsc/system_information.c
+++ b/openbsc/src/libbsc/system_information.c
@@ -791,6 +791,7 @@
 	si13->header.system_information = GSM48_MT_RR_SYSINFO_13;
 
 	si13_default.no_pbcch.rac = bts->gprs.rac;
+	si13_default.no_pbcch.net_ctrl_ord = bts->gprs.net_ctrl_ord;
 
 	ret = rest_octets_si13(si13->rest_octets, &si13_default);
 	if (ret < 0)