Allow modifying the default kernel cmdline allowlist.

BUG=b/538980164
TEST=presubmit
RELEASE_NOTE=None

Change-Id: Ida5918bae2df6e3e5a2c3a6563e68ae31fa7aa9a
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/173026
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Kevin Berry <kpberry@google.com>