sign_official_build.sh: add an update payload operation
sign_official_build.sh: add an update payload operation

This enables the signer script to sign update payloads.

BUG=chromium-os:34521
TEST=`./sign_official_build.sh update_payload testcase.sha256 . foo` produced a signed foo file
BRANCH=None

Change-Id: I27a9de89e760427251538deec38161944388a152
Reviewed-on: https://gerrit.chromium.org/gerrit/33535
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Reviewed-by: Gaurav Shah <2987db6a10bcfe664554a372d01e80df7fd325a3@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

acorn-marvell/vboot_reference,marineam/seismograph,ccaapton/vboot_reference,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,ccaapton/vboot_reference,ccaapton/vboot_reference,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,ionave/seismograph,coreboot/vboot,coreboot/vboot,coreboot/vboot,acorn-marvell/vboot_reference,ionave/seismograph,ionave/seismograph,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,marineam/seismograph,marineam/seismograph,marineam/seismograph,ccaapton/vboot_reference,acorn-marvell/vboot_reference,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference