sys.exit will cause finally statements and other code to run at exit. Since
we're using os.fork() here, os._exit() is apprioriate in this codepath.
(From OE-Core rev: ec08498ff29de9ccd23be88b9d7af3dab6bbb81e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>