license_image.bbclass: Create symlink to the image license manifest dir

license_image.bbclass: Create symlink to the image license manifest dir

In the LICENSE_DIRECTORY each time an image recipe is built a new directory
is created to hold the image license manifests.

By creating a symlink to the most recent created image license manifest
directory, we make things easier for users to collect and pack the data.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>