build-system: Add download.fonosfera.org as a mirror.
This allows Fon to offer specific upstream tarballs as a backup in case
upstream stops providing them. In particular, this is needed because
kernel.org stopped providing the 184.108.40.206 sources for an unknown reason
(might be an error, but nobody responded to inquiries).
This change means that a clean build should work again, without having
to obtain the kernel sources from somewhere first.