Orderfile: Add device flag to orderfile generator.

Previously, the profiling tools selected the first connected
devices. This gives inconsistent and surprising results if there is more
than one device connected. This change asserts that either exactly one
device is connected, or the device is explicitly specified via a new
--device flag.

Bug: 843561
Change-Id: I51e310b83028876936bc8e042d96c9e16044360b
Reviewed-on: https://chromium-review.googlesource.com/1124843
Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576012}