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/include/buflib.h

@@ -103,6 +103,12 @@ struct buflib_callbacks {
      * at least shrinkable
      */
     int (*shrink_callback)(int handle, unsigned hints, void* start, size_t old_size);
+    /**
+     * This is called when special steps must be taken for synchronization
+     * both before the move_callback is called and after the data has been
+     * moved.
+     */
+    void (*sync_callback)(int handle, bool sync_on);
 };
 
 #define BUFLIB_SHRINK_POS_MASK ((1<<0|1<<1)<<30)