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/e1_config.c b/openbsc/src/libbsc/e1_config.c
index 8910d21..d57dec5 100644
--- a/openbsc/src/libbsc/e1_config.c
+++ b/openbsc/src/libbsc/e1_config.c
@@ -179,7 +179,7 @@
 
 	/* skip signal link initialization, this is done later for these BTS. */
 	if (bts->type == GSM_BTS_TYPE_NANOBTS ||
-	    bts->type == GSM_BTS_TYPE_OSMO_SYSMO)
+	    bts->type == GSM_BTS_TYPE_OSMOBTS)
 		return e1inp_line_update(line);
 
 	/* OML link */