Use sleep(0) instead of yield() in SDL_WaitAudio

This allows CPU load to drop below 100%, which is good for anything
that scales frequency and voltage based on CPU load. Also conserves
some energy by letting the core go idle if there aren't any available
buffers.

Change-Id: I9385ac9e030f97010b12eb825875a900463ab0ac

apps/plugins/sdl/src/audio/rockbox/SDL_rockboxaudio.c

@@ -127,7 +127,7 @@ static void ROCKBOXAUD_WaitAudio(_THIS)
             }
         }
 
-        rb->yield();
+        rb->sleep(0);
     }
 }