cos_kernel_args: Allow bitfield file to be an array

This allows encoding arbitrarily long bitfields. For now, we still set
an arbitrary limit of 1024 bits, but this should be good enough.

BUG=None
TEST=presubmit

Change-Id: I9400789b682d807f626b7f69d125a9d9c661f3ff
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/173546
Reviewed-by: Miri Amarilio <mirilio@google.com>
Tested-by: Kevin Berry <kpberry@google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>