image_signing: fix zeroing free space
image_signing: fix zeroing free space

We are not zeroing the free space properly before signing official images. This
patch fixes it by using fstrim instead of dd. More info can be found in
CL:751809.

BRANCH=none
BUG=chromium:780291
TEST=used sign_official_build.sh to sign two recovery images (these images produced <zero-space> file) with dev keys. Then generated delta update between the two new images. This time there was no <zero-space> file between the two images.

Change-Id: Ib97fb206f5c8bcfd97c43d075990c7fcdaad6f7e
Reviewed-on: https://chromium-review.googlesource.com/848237
Commit-Ready: Amin Hassani <6eb7d1420fa042af18129aa7faed43f8a0997890@chromium.org>
Tested-by: Amin Hassani <6eb7d1420fa042af18129aa7faed43f8a0997890@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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