bitbake: knotty: Fix for the Second Keyboard Interrupt

bitbake: knotty: Fix for the Second Keyboard Interrupt

Fixed:
$ rm -fr tmp-glibc/cache/default-glibc/qemux86/x86_64/bb_cache.dat* ; bitbake -p
Press the first Ctrl-C when the parsing process is at about 50%:

Keyboard Interrupt, closing down...

Then presss the second Ctrl-C:

  File "/path/to/bitbake/bitbake/lib/bb/ui/knotty.py", line 619, in main
    event = eventHandler.waitEvent(0.25)
  File "/path/to/bitbake/lib/bb/server/process.py", line 591, in waitEvent
    self.eventQueueNotify.wait(delay)
  File "/usr/lib/python3.5/threading.py", line 549, in wait
    signaled = self._cond.wait(timeout)
  File "/usr/lib/python3.5/threading.py", line 297, in wait
    gotit = waiter.acquire(True, timeout)
KeyboardInterrupt

Capture the second KeyboardInterrupt during stateShutdown is running can fix
the problem. There may be still tracebacks for the third KeyboardInterrupt, but
I'm leaning to not fix it since we aimed for supporting 2 KeyboardInterrupts
only.

(Bitbake rev: 8c26b451f22193ef1c544e2017cc84515566c1b8)

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