buildman: Deal nicely with invalid build-status file

The 'done' files created by buildman may end up being empty if buildman
runs out of disk space while writing them. This error is then persistent,
since even if disk space is reclaimed and the build retries, the empty
file causes an exception in the builder thread.

Deal with this silently by doing a rebuild.

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