ca-certificates: create symbolic link for certificate hashes

Implementing "add-cert.sh" functionality described at
http://wiki.openwrt.org/doc/howto/wget-ssl-certs into Makefile
otherwise you need to create symbolic links for certificate hashes
yourself.

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42660 3c298f89-4303-0410-b956-a3cf2f4a3e73