Notify the buffering thread when a handle is added, so it can go into filling mode. This is some sort of a replacement for the ATA idle callback. It will interrupt an ongoing buffering process (buffer_handle), but not for long enough to be a problem.
Should fix a problem reported by Dave Hooper where inserting tracks into the playlist would flush the buffer and not refill it, sometimes causing the inserted tracks to be skipped.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@16295 a1c6a512-1295-4272-9138-f99709370657