license_image.bbclass: use canonical name for license files

When copying license files to the image rootfs, i.e to
/usr/share/common-licenses, a canonical name of a license should be
used, otherwise duplicated files end up in common-licenses directory.

For example, GPL-2.0 license according to conf/license.conf can be
referenced in recipes as GPL-2, GPLv2, and GPLv2.0. If a license name is
used directly, we end up with three files in the rootfs with the same
content. If a canonical name used instead, then each license gets copied
only once.

Signed-off-by: Vyacheslav Yurkov <Vyacheslav.Yurkov@bruker.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>