buffet: Add device state D-Bus property

This is part of Privet protocol, device should export state over HTTP.
Other daemons may also need this information.
In particular privetd may read availability of local discovery form
there.

BUG=brillo:427
TEST=`FEATURES=test emerge-gizmo buffet`

Change-Id: I2688a1e2f02e25952459badeeaf9dbd4ca84cf55
Reviewed-on: https://chromium-review.googlesource.com/273524
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
3 files changed
tree: d03468037614fa7d784a076b97e062659bca0bfd
  1. buffet/