package/cdrkit: download from alternate site

www.cdrkit.org looks dead, the cdrkit archive always downloads from the
buildroot.org mirror. This patch points the downloader to the Debian
archive site.

Signed-off-by: Karoly Kasza <kaszak@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>