scripts: increase selftest layout kernel partition size

Increase KERN-A and KERN-B size in 'selftest' layout to 64 MiB to accommodate larger kernels in kselftest builds.

BUG=b/526629383
TEST=presubmit
RELEASE_NOTE=None

Change-Id: Ib66807501f26f3b2d15d44c7c82c50f2a20ecee7
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/173566
Reviewed-by: Angel Adetula <angeladetula@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>