sdl: fix freeze on startup on PP targets

Using the coprocessor was a good idea in theory, but didn't actually work.
This moves all SDL threads to the main core, which isn't ideal, but at
least works. Additionally, this also adds some good-practice stuff such as
setting the default sample rate on exit.

Change-Id: If1636b33d439000ec7c4e50f46e8d002708d3121

apps/plugins/sdl/SDL_mixer/mixer.c

@@ -412,7 +412,8 @@ int Mix_OpenAudio(int frequency, Uint16 format, int nchannels, int chunksize)
 			return(0);
 		}
 		while ( audio_opened ) {
-			Mix_CloseAudio();
+                    Mix_CloseAudio();
+                    rb->yield();
 		}
 	}