legal-info: add hash for our own license file

This silences the annoying warning that there is no hash file for our
own COPYING file.

Also change the message so that it is more obvious what we're doing.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>