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/system_information.c b/openbsc/src/libbsc/system_information.c
index 360d229..1c16ec0 100644
--- a/openbsc/src/libbsc/system_information.c
+++ b/openbsc/src/libbsc/system_information.c
@@ -470,6 +470,7 @@
 	/* ip.access nanoBTS needs l2_plen!! */
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 	case GSM_BTS_TYPE_HSL_FEMTO:
 		*output++ = (l2_plen << 2) | 1;
 		l2_plen++;
@@ -505,6 +506,7 @@
 	/* ip.access nanoBTS needs l2_plen!! */
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 	case GSM_BTS_TYPE_HSL_FEMTO:
 		*output++ = (l2_plen << 2) | 1;
 		l2_plen++;
@@ -549,6 +551,7 @@
 	/* ip.access nanoBTS needs l2_plen!! */
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 	case GSM_BTS_TYPE_HSL_FEMTO:
 		*output++ = (l2_plen << 2) | 1;
 		l2_plen++;
@@ -585,6 +588,7 @@
 	/* ip.access nanoBTS needs l2_plen!! */
 	switch (bts->type) {
 	case GSM_BTS_TYPE_NANOBTS:
+	case GSM_BTS_TYPE_OSMO_SYSMO:
 	case GSM_BTS_TYPE_HSL_FEMTO:
 		*output++ = (l2_plen << 2) | 1;
 		l2_plen++;