resign_firmwarefd.sh: Fix flag option name
resign_firmwarefd.sh: Fix flag option name

We didn't get bit by this bug because getlong_opt does partial matching
on long option names. So --flag also works.

BUG=none
TEST=resign a test firmware; ensure preamble flag is preserved.
BRANCH=none

Change-Id: Ifd87c627b82468529fe1241be3629198d194027b
Reviewed-on: https://gerrit.chromium.org/gerrit/35350
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Commit-Ready: Gaurav Shah <2987db6a10bcfe664554a372d01e80df7fd325a3@chromium.org>
Tested-by: Gaurav Shah <2987db6a10bcfe664554a372d01e80df7fd325a3@chromium.org>

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