image_signing: fix thinko in payload signing
image_signing: fix thinko in payload signing

There's already a local |output| variable that we clobbered.

BUG=chromium:935628
TEST=cros-signing/signer/signing_unittest.py passes
BRANCH=none

Change-Id: Idde2aa35053ff6bc149f3f4d1df784e25b4fcdc5
Reviewed-on: https://chromium-review.googlesource.com/c/1490651
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot