A builder thread can wedge if the build process wedges. Add a timeout
to the subprocess. Since it was the call to readline() that would block,
we change the process to have DIB write directly to the log. This allows
us to set a timeout in the Popen.wait() call. And we kill the dib
subprocess, as well.
The timeout value can be controlled in the diskimage configuration and
defaults to 8 hours.
Change-Id: I188e8a74dc39b55a4b50ade5c1a96832fea76a7d