binutils: bump the default target version of binutils

When binutils for the host is not built (which is the case when an
external toolchain is used), the version of binutils used is 2.21,
which is quite old. Since we have bumped to 2.22 as the default
version for the host binutils, let's do the same for the target
binutils.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>