Use buflib for the allocation of voice PCM resources.

Buffers are not allocated and thread is not created until the first
call where voice is required.

Adds a different callback (sync_callback) to buflib so that other
sorts of synchonization are possible, such as briefly locking-out the
PCM callback for a buffer move. It's sort of a messy addition but it
is needed so voice decoding won't have to be stopped when its buffer
is moved.

Change-Id: I4d4d8c35eed5dd15fb7ee7df9323af3d036e92b3

firmware/pcm_mixer.c

@@ -406,6 +406,16 @@ void mixer_channel_calculate_peaks(enum pcm_mixer_channel channel,
         *right = peaks->val[1];
 }
 
+/* Adjust channel pointer by a given offset to support movable buffers */
+void mixer_adjust_channel_address(enum pcm_mixer_channel channel,
+                                  off_t offset)
+{
+    pcm_play_lock();
+    /* Makes no difference if it's stopped */
+    channels[channel].start += offset;
+    pcm_play_unlock();
+}
+
 /* Stop ALL channels and PCM and reset state */
 void mixer_reset(void)
 {