binutils: remove support for binutils 2.25

Now that we have switched to binutils 2.27 as the default binutils
version, it's time to get rid of binutils 2.25. So this commit remove
the 2.25 version choice, the hash file entry, the patches, and adds a
Config.in.legacy option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>