fflash: Add flag to skip disabling rootfs verification

This is useful on the rare occurrence to do testing on readonly rootfs.

BUG=None
TEST=go test ./...
TEST=ninja
TEST=bin/integration-test $dut

Change-Id: If63c2878289c8b85969959f41deaf7384ee28ec5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3947886
Tested-by: Li-Yu Yu <aaronyu@google.com>
Reviewed-by: Terry Cheong <htcheong@chromium.org>
Commit-Queue: Li-Yu Yu <aaronyu@google.com>