tools: scons: Add alternate download URL

sourceforge.net sometimes produces the following tarball file while
being behind a HTTP proxy:

We're sorry -- the Sourceforge site is currently in Disaster Recovery
mode, and currently requires
the use of javascript to function.  Please check back later.

Update to a regular FTP location that allows us to complete the
download.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47244 3c298f89-4303-0410-b956-a3cf2f4a3e73