For #1638, #307, config to discard aac for rtc

trunk/conf/full.conf

@@ -335,6 +335,11 @@ vhost rtc.vhost.srs.com {
         #       discard     Discard bframe, maybe cause browser with little problems.
         # default: keep
         bframe      discard;
+        # The strategy for aac audio.
+        #       transcode   Transcode aac to opus.
+        #       discard     Discard aac audio packet.
+        # default: transcode
+        aac         transcode;
     }
 }