One drawback to tinfoil is that once a cooker is created, it will hold
the cooker lock and stop any other bitbake execution against a directory.
Add a shutdown method to tinfoil, allowing other users to use
the build directory after the tinfoil usage is no longer needed.
(Bitbake rev: e44ce85fe551677fc0dcc1da4f789a0c13093ff1)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>