scripts/image_signing: Call futility instead of its symlinks

To reduce the number of binary files needed for the signer, replace
calls to futility symlinks with direct futility calls.

BUG=b:305659154
TEST=cq
BRANCH=none

Disallow-Recycled-Builds: test-failures
Change-Id: Ibf6ee80c68f603510f1ba491d0329c1ac633f2e0
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/5268241
Reviewed-by: Julius Werner <jwerner@chromium.org>