license_image.bbclass: Detect broken symlinks

Find and report symlinks which point to a non-existing file.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 81809a1f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>