cos_kernel: Replace StringArray args with StringSlice

This makes it so that values can be passed as either separate arguments
or comma-separated lists.

BUG=None
TEST=presubmit, `cos-kernel build -c lakitu_defconfig,debug.config`

Change-Id: I7466a4811a7e4c7b9f3c7d59dfe7191f4c6e544c
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/195326
Tested-by: Kevin Berry <kpberry@google.com>
AI-Review: cloud-image-release <cloud-image-release@prod.google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: Derek Taylor <ddtaylor@google.com>