This adds a new library on top of subprocess which permits access to
the subprocess output as it is being generated. We can therefore
give the illusion that a process is running independently, but still
monitor its output so that we know what is going on.

It is possible to display output on a terminal as it is generated
(a little like tee). The supplied output function is called with all
stdout/stderr data as it arrives.

Signed-off-by: Simon Glass <sjg@chromium.org>