buildroot

Config.in: use better URL for GNU mirrors

As explained on http://www.gnu.org/prep/ftp.en.html, the network of
GNU mirrors provide a generic address, http://ftpmirror.gnu.org/,
which automatically redirects to an up-to-date and local mirror. This
allows to greatly speed-up downloads, and also reduces the load on the
main GNU server.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>