core/pkg-utils: check hashes of license files

core/pkg-utils: check hashes of license files

This will help catch a change of license even if the filename does
not change.

For now, a missing hash for the license files is not a fatal error, to
let people catch up and add them. When we switch to make it mandatory,
we can simplify the code by just removing the case statement.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>