bitbake: cooker: Cleanup the queue before call process.join()

bitbake: cooker: Cleanup the queue before call process.join()

Fixed:
$ rm -fr tmp-glibc/cache/default-glibc/qemux86/x86_64/bb_cache.dat* ; bitbake -p
Press *one* Ctrl-C when the parsing process is at about 50%, then the processes
are not exited:

Keyboard Interrupt, closing down...

Timeout while waiting for a reply from the bitbake server

It hangs at process.join(), according to:

https://docs.python.org/3.7/library/multiprocessing.html

Cleanup the queue before call process.join() can fix the problem.

(Bitbake rev: 3eddfadd19b2ce4c061861abf0c340e3825b41ff)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>