login: Implement StartTPMFirmwareUpdate DBus call.

Implement a new DBus call that will update the TPM firmware update
mode in VPD if applicable and permitted.

BUG=chromium:742985
TEST=None
CQ-DEPEND=CL:575244

Change-Id: I7d472c1e53b89d7a44ba21bcad4ec5ab9638ce32
Reviewed-on: https://chromium-review.googlesource.com/574857
Commit-Ready: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
5 files changed