license_image.bbclass: Fix symlink to generic license files

Link to the canonical filename of a license as only this one exists.

Fixes commit 670fe71d
[license_image.bbclass: use canonical name for license files].

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>