gsm_bts: add version and variant details

* add version string to gsm_bts
* add PCU version string to gsm_bts
* rename GSM_BTS_TYPE_OSMO_SYSMO -> GSM_BTS_OSMOBTS to avoid confusion
  between BTS model and variant
* add variant enum to gsm_bts_model using enum with variants for each
  hw vendor of OsmoBTS
* show connected PCU version (if available) in vty via 'show bts'

This will come in handy when logging details regarding particular BTS
reported via OML, see:

Related: OS#1614

Change-Id: I6710d53115f34634a7b70969cc05fd5c72ff8ab2
diff --git a/openbsc/src/libbsc/bts_sysmobts.c b/openbsc/src/libbsc/bts_sysmobts.c
index e1bf661..e4b6cdc 100644
--- a/openbsc/src/libbsc/bts_sysmobts.c
+++ b/openbsc/src/libbsc/bts_sysmobts.c
@@ -46,7 +46,7 @@
 {
 	model_sysmobts = bts_model_nanobts;
 	model_sysmobts.name = "sysmobts";
-	model_sysmobts.type = GSM_BTS_TYPE_OSMO_SYSMO;
+	model_sysmobts.type = GSM_BTS_TYPE_OSMOBTS;
 
 	model_sysmobts.features.data = &model_sysmobts._features_data[0];
 	model_sysmobts.features.data_len =