The unionfs has been disabled for more than a year and it's not going
to be used any more.
This patch cleans up the unionfs related code.
[YOCTO #4761]
[YOCTO #1487]
(From OE-Core rev: ba5e437bc7335468a70ea293496f78e1a9d66287)
Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>