We currently wait for the child to exit but we don't flush the buffers.
This can mean the output ends up out of sync and the END: line isn't at
the end of the logs.

We've recently seen a lot of issues related to this on the autobuilder.
Add in a flush call for all fds to ensure buffers are in sync. This
does appear to improve warnings on the autobuilder now we started detecting
the issue.

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