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/libcommon/gsm_data.c b/openbsc/src/libcommon/gsm_data.c
index a235c9d..5c58bf3 100644
--- a/openbsc/src/libcommon/gsm_data.c
+++ b/openbsc/src/libcommon/gsm_data.c
@@ -189,6 +189,7 @@
 	{ GSM_BTS_TYPE_RBS2000,	"rbs2000" },
 	{ GSM_BTS_TYPE_HSL_FEMTO, "hsl_femto" },
 	{ GSM_BTS_TYPE_NOKIA_SITE, "nokia_site" },
+	{ GSM_BTS_TYPE_OSMO_SYSMO, "sysmobts" },
 	{ 0,			NULL }
 };
 
@@ -338,6 +339,7 @@
 	case GSM_BTS_TYPE_HSL_FEMTO:
 		bts->c0->rsl_tei = 0;
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 		/* Set the default OML Stream ID to 0xff */
 		bts->oml_tei = 0xff;
 		bts->c0->nominal_power = 23;