Introduce BTS_FEAT_ETWS_PN for communicating ETWS PN capability

As 3GPP doesn't specify how the BSC shall communicate ETWS Primary
Notifications over Abis/RSL, we have to use a vendor-specific RSL
message for this.  And in order to know if the peer supports this
feature, we introduces BTS_FEAT_ETWS_PN.

Change-Id: I89c24a81ada6627694a9632e87485a61cbd3e680
Related: OS#4046, OS#4047

include/osmocom/gsm/bts_features.h

@@ -23,6 +23,7 @@ enum osmo_bts_features {
 	BTS_FEAT_SPEECH_F_EFR,
 	BTS_FEAT_SPEECH_F_AMR,
 	BTS_FEAT_SPEECH_H_AMR,
+	BTS_FEAT_ETWS_PN,
 	_NUM_BTS_FEAT
 };
 

src/gsm/bts_features.c

@@ -38,6 +38,7 @@ const struct value_string osmo_bts_features_descs[] = {
 	{ BTS_FEAT_SPEECH_F_EFR,	"Fullrate speech EFR" },
 	{ BTS_FEAT_SPEECH_F_AMR,	"Fullrate speech AMR" },
 	{ BTS_FEAT_SPEECH_H_AMR,	"Halfrate speech AMR" },
+	{ BTS_FEAT_ETWS_PN,		"ETWS Primary Notification via PCH" },
 	{ 0, NULL }
 };