image_signing: do not delete output when signing loem firmware
image_signing: do not delete output when signing loem firmware

Rather than use the existence of the output as a marker for running the
gbb step, key off the loem index.  We want to run it the first time and
not bother after that.

BUG=chrome-os-partner:44227
BRANCH=None
TEST=signer can still sign loem keysets

Change-Id: I26e9ccaf1333f769d6993a8e0d84c63644bb2597
Reviewed-on: https://chromium-review.googlesource.com/298980
Reviewed-by: David Riley <a1c52a99f156ff41f0ea478f29291144551a02f6@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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