use avr32linux.org mirror

The avr32linux.org mirror has vanished and is replaced by a spam site
that returns HTML pages for whatever URL you request from it.  So the
download helper thinks that download has succeeded, while actually it
failed.

Fortunately, there is still a mirror of the site alive, so we can use
that one.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>