toolchain-utils: Use the chromeos_image value computed initially.

When reimaging with a local image, use the chromeos_image value computed
during the initial analysis, instead of attempting to recompute it at each
use. The recomputation at the verification stage was also incorrect.

BUG=b:79376118
TEST=Tested with crosperf and two local images. The script correctly switches
between images and it doesn't fail during the reimaging step.

Change-Id: Ie40ed52999ecbebb4e2f5dd3e0aa78a18482533a
Reviewed-on: https://chromium-review.googlesource.com/1058181


Commit-Ready: Gabriel Marin <gmx@chromium.org>
Tested-by: Gabriel Marin <gmx@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>