image_signing: work around shar invocation changing
image_signing: work around shar invocation changing

The shar utility changed the flag name and newer versions no longer
accept -w.  Mung the shellball on the fly to use the newer flag.

BUG=chrome-os-partner:33719
TEST=signing old firmware shellball inside chroot passes
BRANCH=none

Change-Id: If5c2da3062bd72062baa779bb26ea56304c31558
Reviewed-on: https://chromium-review.googlesource.com/241064
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

acorn-marvell/vboot_reference,coreboot/vboot,coreboot/vboot,coreboot/vboot,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,acorn-marvell/vboot_reference