[HSL] initial support for the HSL 2.75G Femtocell

The HSL Femtocell seems to be a poor man implementation of the
ip.access Abis/IP protocol, but cutting corners wherever possible.

We try to workaround those corners wherever possible...
diff --git a/openbsc/src/libbsc/abis_nm.c b/openbsc/src/libbsc/abis_nm.c
index 45db8ba..0e7fc8d 100644
--- a/openbsc/src/libbsc/abis_nm.c
+++ b/openbsc/src/libbsc/abis_nm.c
@@ -1080,6 +1080,12 @@
 	case NM_MT_IPACC_RESTART_NACK:
 		dispatch_signal(SS_NM, S_NM_IPACC_RESTART_NACK, NULL);
 		break;
+	case NM_MT_SET_BTS_ATTR_ACK:
+		/* The HSL wants an OPSTART _after_ the SI has been set */
+		if (mb->trx->bts->type == GSM_BTS_TYPE_HSL_FEMTO) {
+			abis_nm_opstart(mb->trx->bts, NM_OC_BTS, 255, 255, 255);
+		}
+		break;
 	}
 
 	abis_nm_queue_send_next(mb->trx->bts);