[toolchain/binutils] set PKG_SOURCE_URL to http://mirror2.openwrt.org/sources

[toolchain/binutils] set PKG_SOURCE_URL to mirror2.openwrt.org/sources

The source tarballs of binutils version 2.20.1 / 2.21.1 got repackaged and replaced
by new versions back in 2011. This was due to missing license files* - the change
didn't affect the actual source code.
Since we can't just change the md5sum and replace the tarball (as it might break
previous releases/builds), the "old" tarballs got put on our mirror servers.
As they're the only ones providing "valid" archives, directly point to them
instead of trying the GNU ones before.

*http://sourceware.org/ml/binutils/2011-08/msg00198.html

git-svn-id: svn://svn.openwrt.org/openwrt/branches/backfire@32037 3c298f89-4303-0410-b956-a3cf2f4a3e73