rtp: Add handling of BFI (Bad Frame Indicatior) of received TRAU frames

If a bad TRAU frame is received, it is forwarded to MNCC application
as GSM_BAD_FRAME. The application can now handle the GAP of missing
audio. (e.g. by extrapolation)

If TRAU frames are forwarded via RTP, bad frames are dropped, but frame
counter and timestamp of RTP sender state is incremented.

Conflicts:
	openbsc/src/libtrau/rtp_proxy.c

[hfreyther: Merge without testcase, fix typo]
diff --git a/openbsc/src/libtrau/trau_mux.c b/openbsc/src/libtrau/trau_mux.c
index 3831a80..b37c765 100644
--- a/openbsc/src/libtrau/trau_mux.c
+++ b/openbsc/src/libtrau/trau_mux.c
@@ -245,7 +245,10 @@
 		i++;
 		j++;
 	}
-	frame->msg_type = GSM_TCHF_FRAME;
+	if (tf->c_bits[11]) /* BFI */
+		frame->msg_type = GSM_BAD_FRAME;
+	else
+		frame->msg_type = GSM_TCHF_FRAME;
 	frame->callref = callref;
 	msgb_put(msg, sizeof(struct gsm_data_frame) + 33);