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/bts_init.c b/openbsc/src/libbsc/bts_init.c
index 693eca8..ae41ecc 100644
--- a/openbsc/src/libbsc/bts_init.c
+++ b/openbsc/src/libbsc/bts_init.c
@@ -25,6 +25,7 @@
 	bts_model_nanobts_init();
 	bts_model_hslfemto_init();
 	bts_model_nokia_site_init();
+	bts_model_sysmobts_init();
 	/* Your new BTS here. */
 	return 0;
 }