fix parallel build failures

fix parallel build failures

When building in parallel, the btreplay/btrecord and btreplay/btreplay
targets cause make to kick off two jobs for `make -C btreplay` and they
sometimes end up clobbering each other.  We could fix this by making one
a dependency of the other, but it's a bit cleaner to refactor things to
be based on subdirs.  This way changes in subdirs also get noticed:
  $ touch btreplay/*.[ch]
  $ make
  <btreplay is now correctly updated>

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>