Make BTS type and variant converters shareable

* move value_string definition and corresponding functions for BTS type
  to shared header to make it re-usable by OsmoBTS
* use consistent function naming
* add similar functions for BTS variant
* add enum to be used by OML Attribute Reporting to distinguish between
  type, variant and other info

Change-Id: Ida94725a6fce968443541e3526f48f13758031fd
Related: OS#1614
diff --git a/openbsc/src/libbsc/bsc_vty.c b/openbsc/src/libbsc/bsc_vty.c
index 195fd6a..2794d85 100644
--- a/openbsc/src/libbsc/bsc_vty.c
+++ b/openbsc/src/libbsc/bsc_vty.c
@@ -1614,7 +1614,7 @@
 	struct gsm_bts *bts = vty->index;
 	int rc;
 
-	rc = gsm_set_bts_type(bts, parse_btstype(argv[0]));
+	rc = gsm_set_bts_type(bts, str2btstype(argv[0]));
 	if (rc < 0)
 		return CMD_WARNING;