image_signing: fix breakage from dropping <R16 support

Commit 16ceb9625ed13b0da4ae6306f9187b672b9b382f dropped support for old
versions, but it also accidentally dropped the salt= setting which ended
up breaking newer recovery kernels.  Restore that line and drop an unused
var from the old code path.

BRANCH=None
BUG=chromium:891015, chromium:891764
TEST=running `./sign_official_build.sh verify` against the images in crbug.com/891764 works again

Change-Id: I8ae619c9243f9c2638962ae439b9df5090d6c535
Reviewed-on: https://chromium-review.googlesource.com/1376831
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
1 file changed