scripts: add selftest layout for kselftest.

Added a common layout 'selftest' for the kselftest image.

BUG=b/526629383
TEST=presubmit, validation test
RELEASE_NOTE=None

Change-Id: I86c107f7c4aeca732c3c4f86a6dabb6cd8eaaa3c
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/170385
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Kevin Berry <kpberry@google.com>