The package might be empty while pthread functions are being provided by
libc, so we need set ALLOW_EMPTY with it or it will break do_rootfs task.
(From OE-Core rev: 53efd76f7955375986a036924513bb374a918f0b)
Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>