servo v4 support
[servod] Add in post_init functionality for servod.

We need a way to detect on a servo v4 if a servo micro is present and is
connected to the servo v4.  This adds in the scaffolding to do all of
that, the remaining worklist:

- detect if servo micro is behind servo v4
- init servo micro interfaces

BUG=chromium:599538
TEST=locally start servod on a servo_host for servo v4 and can properly
initialize all interfaces on servo v4.

Change-Id: Ifeba67a7a0dc532e127244fe6c16ad96f96c79c3
Reviewed-on: https://chromium-review.googlesource.com/359982
Commit-Ready: Kevin Cheng <kevcheng@chromium.org>
Tested-by: Kevin Cheng <kevcheng@chromium.org>
Reviewed-by: Kevin Cheng <kevcheng@chromium.org>
2 files changed