Add a 'power_state' control to selected board overlays.

This adds a new software control called 'power_state' that provides
the necessary behaviors needed for Autotest to perform repair via
servo.

This is an initial phase to support just the x86-based boards
currently supported by Autotest.  The support here doesn't include
new x86-boards not already supported in the test lab, nor does it
include any ARM boards.

BUG=chromium:326738
TEST=tested all power_state settings against link hardware

Change-Id: Idf1dbab1975e20155a1300730c57c3e0266c585e
Reviewed-on: https://chromium-review.googlesource.com/186175
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
12 files changed