scripts: support for optional hypervisor kernel.

BUG=b:145357269
TEST=build and update eve-manatee with/without hypervisor.

Change-Id: I0e8aa49f7b13894b598feac444ad2f11622532b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2391302
Tested-by: Tomasz Jeznach <tjeznach@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Allen Webb <allenwebb@google.com>