buildman: Write output files when using -w

buildman: Write output files when using -w

At present buildman does not write its own output files (err, done, the
environment) when using -w. However this is useful for when the build is
run with -s to check it.

In fact ProduceResultSummary() reads the result from those files rather
than using the 'result' info directly. So ProcessResult() does not work
with -w at present. It does not print any output.

Fix this by writing output files even when -w is used.

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