conf/bitbake.conf: use gnu mirror instead of main server

conf/bitbake.conf: use gnu mirror instead of main server

ftp.gnu.org is the main server of the GNU project, however download speed
can vary greatly based on one's location.

Using ftpmirror.gnu.org should redirect the request to the closest up-to-date mirror,
which should result sometimes in significantly faster download speed, depending
on one's location. This should also distribute the traffic more across the mirrors.

This information was sourced from https://www.gnu.org/prep/ftp.html .

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d8c6f01d7467e018aa0ed27a87850d9e4434a47a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>