cos-kernel: move config fragments to separate files

Move kernel configuration fragments and their descriptions from the
eclass to individual .config files. This improves the maintainability
and readability of cos-kernel.eclass by replacing long configuration
strings with a dynamic loader.

BUG=
TEST=presubmit, local kernel emerge
RELEASE_NOTES=None

Change-Id: I125f490c77ecccf9a6b0ef87d5845c8595e13e0b
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/139286
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Kevin Berry <kpberry@google.com>