Add RTP support for EFR

This is just the minimal support to receive and send EFR codec
RTP frames.

We are missing the code to convert TRAU frames in EFR format!
diff --git a/openbsc/src/mncc.c b/openbsc/src/mncc.c
index a5efc73..f53b15d 100644
--- a/openbsc/src/mncc.c
+++ b/openbsc/src/mncc.c
@@ -83,7 +83,7 @@
 	{"MNCC_FRAME_DROP",	0x0202},
 	{"MNCC_LCHAN_MODIFY",	0x0203},
 
-	{"GSM_TRAU_FRAME",	0x0300},
+	{"GSM_TCH_FRAME",	0x0300},
 
 	{NULL, 0} };