vboot: support signing with pkcs11 private key

The subcommands of futility using the vb2_private_key struct now could
read the pkcs11 key and use it for signing process.

BRANCH=none
BUG=b:294369070
TEST=cros_run_unit_tests --host --packages vboot_reference
TEST=manual run `futility vbutil_kernel` with pkcs11 key

Cq-Depend: chromium:4900142
Signed-off-by: Ching-Kang Yen <chingkang@google.com>
Change-Id: I971880fe4ff0c94d4ff259f2bd7b3c92cf45cc6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/4798534
Tested-by: Ching-Kang Yen <chingkang@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: John L Chen <zuan@chromium.org>
Commit-Queue: Ching-Kang Yen <chingkang@chromium.org>