u-boot-fw-utils: Fix broken makefile in v2017.11.

u-boot-fw-utils: Fix broken makefile in v2017.11.

See the patch for details. This patch has already been applied
upstream, but we need it for v2017.11.

Upstream-Status: Accepted [http://git.denx.de/?p=u-boot.git;a=commit;h=ded84f90a1066eef5f34daa4539273de64f7b811]

(From OE-Core rev: bb6a45bc7e122c61386d0657e7c97895d1697bd7)

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>