[vty] Allow to set the RACH NM attributes on a per BTS basis

Be able to tune the RACH settings of the BTS via the vty interface,
by default they are initialized to -1 which means we will use the
content of the static array (BTS default) and can be changed via
the VTY interface. I have verified the setting on the nanoBTS with
wireshark and I have tested writing the config file.
diff --git a/openbsc/src/gsm_data.c b/openbsc/src/gsm_data.c
index 4d8fa17..9cb5647 100644
--- a/openbsc/src/gsm_data.c
+++ b/openbsc/src/gsm_data.c
@@ -221,6 +221,8 @@
 	}
 	bts->c0->ts[0].pchan = GSM_PCHAN_CCCH_SDCCH4;
 
+	bts->rach_b_thresh = -1;
+	bts->rach_ldavg_slots = -1;
 	llist_add_tail(&bts->list, &net->bts_list);
 
 	return bts;