vboot: use vboot2 for storing kernel rollback versions

Migrate to using vboot2 shared data fields for storing
kernel rollback versions:

  kernel_version_tpm_start(1) --> kernel_version_secdata(2)
  kernel_version_tpm(1) --> kernel_version(2)
  kernel_version_lowest(1) --> [removed]

Also remove VBSD from tests which no longer need it.

BUG=b:124141368, chromium:1038260
TEST=make clean && make runtests
BRANCH=none

Change-Id: I26c2ccede5fba52e1477b625ef5fc6181f60aadf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2053179
Tested-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
10 files changed