BSC: introduce new "sysmobts" BTS model

so far, osmo-bts/sysmobts used to be entered as "sysmobts" type in the
configuration file.  However, there are some differences in the
protocol/behaviour and we should reflect that by a new BTS plugin (with
lots of code reuse from the nanobts driver).
diff --git a/openbsc/src/libbsc/abis_nm.c b/openbsc/src/libbsc/abis_nm.c
index 6568b53..673f43e 100644
--- a/openbsc/src/libbsc/abis_nm.c
+++ b/openbsc/src/libbsc/abis_nm.c
@@ -610,6 +610,7 @@
 
 	switch (bts_type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 		rc = abis_nm_rx_ipacc(mb);
 		abis_nm_queue_send_next(sign_link->trx->bts);
 		break;
@@ -1531,6 +1532,9 @@
 			break;
 		}
 		return -EINVAL;
+	case GSM_BTS_TYPE_OSMO_SYSMO:
+		/* no known restrictions */
+		return 0;
 	default:
 		/* unknown BTS type */
 		return 0;