build_kernel_image: add warning for kernel parameters w.r.t. signing

BUG=None
TEST=Comment Only

Change-Id: I03fd7e875686c2f3ce78919d5f893cb2ecb10525
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2541322
Tested-by: George Engelbrecht <engeg@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Commit-Queue: George Engelbrecht <engeg@google.com>