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/e1_config.c b/openbsc/src/libbsc/e1_config.c
index cda1318..d3dff48 100644
--- a/openbsc/src/libbsc/e1_config.c
+++ b/openbsc/src/libbsc/e1_config.c
@@ -185,6 +185,7 @@
 
 	/* skip signal link initialization, this is done later for these BTS. */
 	if (bts->type == GSM_BTS_TYPE_NANOBTS ||
+	    bts->type == GSM_BTS_TYPE_OSMO_SYSMO ||
 	    bts->type == GSM_BTS_TYPE_HSL_FEMTO)
 		return e1inp_line_update(line);