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/bsc_vty.c b/openbsc/src/libbsc/bsc_vty.c
index 413eac4..f306488 100644
--- a/openbsc/src/libbsc/bsc_vty.c
+++ b/openbsc/src/libbsc/bsc_vty.c
@@ -517,6 +517,7 @@
 	}
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 		vty_out(vty, "  ip.access unit_id %u %u%s",
 			bts->ip_access.site_id, bts->ip_access.bts_id, VTY_NEWLINE);
 		vty_out(vty, "  oml ip.access stream_id %u line %u%s",