Fix backup and kernel download url.

Remove duplicated slash from backup and kernel download url,
e.g. http://www.kernel.org/pub//linux/kernel/v3.x/linux-3.13.3.tar.xz



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>