cve-check: Add helper for symlink handling

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5046d54d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>