classes/mirrors: use geo-located kernel.org mirrors

classes/mirrors: use geo-located kernel.org mirrors

We use the kernel.org mirrors for a number of projects: obviously the
kernel, but also the GNU tarballs are fetched from there too.

However, mirrors.kernel.org does not have any geo-proximity DNS magic
and will always resolve to the primary server on in west coast USA,
which is far from ideal if you're not near there.

Switch the mirror URLs to mirrors.edge.kernel.org, which does actually
resolve to a closer server.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>