oe/utils.py: Fix thread leakage in ThreadPool

oe/utils.py: Fix thread leakage in ThreadPool

In order to fix Thread leakage caused by not call join() in Threads,

Pass num_tasks in ThreadPool for add all the tasks into a Queue this
enable catch of Queue.Empty exception and exit the threads.

classes/sstate.bbclass: Change checkstatus function to match new
ThreadPool operation.

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>