In the forked child, we may use multiprocessing. There is only one event
pipe to the worker controlling process and if we're unlucky, multiple
processes can write to it at once corrupting the data by intermixing it.

We don't see this often but when we do, its quite puzzling. I suspect it
only happens in tasks which use multiprocessng (do_rootfs, do_package)
and is much more likely to happen when we have long messages, usually many
times PAGE_SIZE since PAGE_SIZE writes are atomic. This makes it much more
likely within do_roofs, when for example a subprocess lists the contents
of a rootfs.

To fix this, we give each child a Lock() object and use this to serialise
writes to the controlling worker.

(Bitbake rev: 3cb55bdf06148943960e438291f9a562857340a3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>