sign_official_build: allow repacking firmware when executed by sudo
sign_official_build: allow repacking firmware when executed by sudo

To prevent execution permissions lost after being copied to /tmp, force adding
a+rx to the staging file.

BUG=chromium-os:20797
TEST=sudo sign_official_build.sh ssd \
     x86-zgb-0.16.1089.0.bin ../../tests/devkeys ssd_image.bin

Change-Id: Ibee12dbb3faea9f6b05600d1343620e0af8633fb
Reviewed-on: http://gerrit.chromium.org/gerrit/8263
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-by: Gaurav Shah <2987db6a10bcfe664554a372d01e80df7fd325a3@chromium.org>
Commit-Ready: Gaurav Shah <2987db6a10bcfe664554a372d01e80df7fd325a3@chromium.org>

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