rust: Check for platform with no commands

Also adds a Dummy platform that we can use for testing from now on.

BUG=chromium:795883
TEST=meson -Darch=amd64 -Duse_cros_config=true out; ninja -C out; MESON_BUILD_ROOT=out cargo build
BRANCH=none

Change-Id: Ie71bc8c48411eb442c84c5ea4a4200eec0d5e683
Reviewed-on: https://chromium-review.googlesource.com/1034389
Commit-Ready: Jason Clinton <jclinton@chromium.org>
Tested-by: Jason Clinton <jclinton@chromium.org>
Reviewed-by: Lann Martin <lannm@chromium.org>
6 files changed