Add gsm_bts_type_variant to gsm_bts struct

Previously it was only in gsm_bts_model which is not initialized on BTS
side. It's more convenient to have it in the struct which is available
to BTS as well.

Change-Id: I54fde8c4ccd5d994af08074f5864446e79a93a25
Related: OS#1614

openbsc/include/openbsc/gsm_data_shared.h

@@ -664,6 +664,7 @@ struct gsm_bts {
 	uint8_t bsic;
 	/* type of BTS */
 	enum gsm_bts_type type;
+	enum gsm_bts_type_variant variant;
 	struct gsm_bts_model *model;
 	enum gsm_band band;
 	char version[MAX_VERSION_LENGTH];