cros_unibuild_convert: add form-factor field

The form-factor field is required by cros_config, make sure it is set
for any device that gets converted to using unibuild.

BUG=b:186608669
TEST=presubmit

Change-Id: I69791d02cbacce77362b50eac8720e6e03fc9728
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2870830
Tested-by: Nikolai Artemiev <nartemiev@google.com>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
1 file changed