scripts: add a utility to fill blank fields of doc/README.scrapyard

We are removing bunch of non-generic boards these days.

Updating doc/README.scrapyard is a really tedious task, but it can
be automated.  I hope this tool will make our life easier.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>