bisect-kit: fix local path for cros flash

Use path relative to chromeos_root. Also updated cros flash version
checks.

BUG=b:185731905
TEST=manually

Change-Id: I69062c140492019e2d0673551bdbcb682fac1241
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2841991
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Auto-Submit: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
2 files changed