Replace all uses of PLUGIN_USE_IRAM with USE_IRAM

PLUGIN_USE_IRAM is almost equivalent to USE_IRAM except
that USE_IRAM might be defined in core, but not plugins.
All remaining uses of PLUGIN_USE_IRAM are inside plugins,
however, so there is no point keeping both defines around.

Change-Id: I6902c85651f3d82b7d19ea32eaa60fc5c19eded7

apps/plugins/pacbox/pacbox.c

@@ -387,7 +387,7 @@ static void start_sound(void)
     if (sound_playing)
         return;
 
-#ifndef PLUGIN_USE_IRAM
+#ifndef USE_IRAM
     /* Ensure control of PCM - stopping music itn't obligatory */
     rb->plugin_get_audio_buffer(NULL);
 #endif