buffet: Populate firmwareVersion BUG=brillo:103 TEST=buffet_BasicDBusAPI confirms this works as intended. Change-Id: I90e3da7f7a4fd7901e4a605413d81cbd14a43d09 Reviewed-on: https://chromium-review.googlesource.com/247234 Tested-by: Christopher Wiley <wiley@chromium.org> Reviewed-by: Alex Vakulenko <avakulenko@chromium.org> Commit-Queue: Christopher Wiley <wiley@chromium.org>