[OML] parse attributes depending on BTS type

Some NM attributes are defined differently depending on
the BTS type.  Having one big nm_att_tlvdef[] table for
all BTS types is no longer sufficient.  This patch

* introduces 'struct gsm_bts_model' to describe a BTS model
* adds definitions of gsm_bts_model for BS-11 and nanoBTS
* changes the abis_nm_tlv_parse() function: include a bts pointer
diff --git a/openbsc/src/vty_interface.c b/openbsc/src/vty_interface.c
index 8d6e202..2654447 100644
--- a/openbsc/src/vty_interface.c
+++ b/openbsc/src/vty_interface.c
@@ -1339,8 +1339,11 @@
       "Set the BTS type\n")
 {
 	struct gsm_bts *bts = vty->index;
+	int rc;
 
-	gsm_set_bts_type(bts, parse_btstype(argv[0]));
+	rc = gsm_set_bts_type(bts, parse_btstype(argv[0]));
+	if (rc < 0)
+		return CMD_WARNING;
 
 	return CMD_SUCCESS;
 }