cve-check: move update_symlinks to a library

Move the function to a library, it could be useful in other places.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit debd37ab)
Signed-off-by: Steve Sakoman <steve@sakoman.com>