Before exiting the UI, unlocks the bitbake.lock owned by cooker; this
way consecutive bitbake executions can lock it again without trouble.
[Yocto #7941]
(Bitbake rev: 69ecd15aece54753154950c55d7af42f85ad8606)
Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>