[BSC] introduce the concept of 'BTS features'

We can then check if a bts supports a certain feature or not.
diff --git a/openbsc/src/bts_ipaccess_nanobts.c b/openbsc/src/bts_ipaccess_nanobts.c
index cb48ea9..92ab41a 100644
--- a/openbsc/src/bts_ipaccess_nanobts.c
+++ b/openbsc/src/bts_ipaccess_nanobts.c
@@ -80,5 +80,11 @@
 
 int bts_model_nanobts_init(void)
 {
+	model_nanobts.features.data = &model_nanobts._features_data;
+	model_nanobts.features.data_len = sizeof(model_nanobts._features_data);
+
+	gsm_btsmodel_set_feature(&model_nanobts, BTS_FEAT_GPRS);
+	gsm_btsmodel_set_feature(&model_nanobts, BTS_FEAT_EGPRS);
+
 	return gsm_bts_model_register(&model_nanobts);
 }