Add psu command

This adds a command for obtaining information about the power supply.
Currently the only subcommand is for finding what type is used.

BUG=chrome-os-partner:23944
BRANCH=none
TEST=not yet...

Change-Id: I1e4a6fa7217231cbec2712f0ee9141aa5e83bd24
Reviewed-on: https://chromium-review.googlesource.com/183753
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
4 files changed