pkgs/os-specific/linux/firmware/bcm43xx: please do not use fetchsvn in base system

Replaced use of fetchsvn with fetchurl. Please note that machines behind a
company firewall usually cannot access svn://-style URLs, which means that
nixos-rebuild is going to fail. HTTP works fine, though.

The URL I used to download the tar.gz archive is probably not stable, or
rather, the tar.gz archive generated by Gitweb at that URL might have a
different checksum every time it's generated. I'm not sure what else to do,
though. Could a kind firmware expert please improve the situation further?

Also, I wonder what is the purpose of the sed expression in the command

    cp $i $out/brcm/$(echo $i | sed 's/\(.*\.fw\).*/\1/')

...? The downloaded directory doesn't seem to contain any files that would
match that expression?

svn path=/nixpkgs/trunk/; revision=30010