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/abis_nm.c b/openbsc/src/libbsc/abis_nm.c
index 651ca02..56b6fcf 100644
--- a/openbsc/src/libbsc/abis_nm.c
+++ b/openbsc/src/libbsc/abis_nm.c
@@ -738,7 +738,7 @@
 
 	switch (bts_type) {
 	case GSM_BTS_TYPE_NANOBTS:
-	case GSM_BTS_TYPE_OSMO_SYSMO:
+	case GSM_BTS_TYPE_OSMOBTS:
 		rc = abis_nm_rx_ipacc(mb);
 		abis_nm_queue_send_next(sign_link->trx->bts);
 		break;
@@ -1723,7 +1723,7 @@
 		}
 		*reason = "Unknown combination";
 		return -EINVAL;
-	case GSM_BTS_TYPE_OSMO_SYSMO:
+	case GSM_BTS_TYPE_OSMOBTS:
 		/* no known restrictions */
 		return 0;
 	default: