binutils: use binutils 2.24 by default on the target

When binutils is not built for the host, binutils.mk decides of the
version to be used when building binutils for the target. We're still
using the old 2.22 binutils version, but it's time to bump up to 2.24,
which we already use as the default version for the host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>