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/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index e596b21..c6a5436 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -1500,6 +1500,7 @@
 	// todo: map between different bts types
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 		if (!ipacc_rtp_direct) {
 			/* connect the TCH's to our RTP proxy */
 			rc = rsl_ipacc_mdcx_to_rtpsock(lchan);
@@ -1571,6 +1572,7 @@
 
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 		if (ipacc_rtp_direct) {
 			LOGP(DCC, LOGL_ERROR, "Error: RTP proxy is disabled\n");
 			return -EINVAL;
@@ -2879,6 +2881,7 @@
 		bts = trans->conn->lchan->ts->trx->bts;
 		switch (bts->type) {
 		case GSM_BTS_TYPE_NANOBTS:
+		case GSM_BTS_TYPE_OSMO_SYSMO:
 			if (!trans->conn->lchan->abis_ip.rtp_socket) {
 				DEBUGP(DMNCC, "TCH frame to lchan without RTP connection\n");
 				return 0;