Enforce block_devmode flag in chromeos_startup.

When the block_devmode crossystem flag has been set by the device
owner, we should block boots in developer mode.

BUG=chromium:375775
TEST=Manual.
CQ-DEPEND=CL:201640

Change-Id: I6ec90bf6377794f22ed08b40b750c3329eb183e6
Reviewed-on: https://chromium-review.googlesource.com/201650
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
2 files changed