update_kernel.sh: Add some narrative help text

I'm updating the kernel_development.md docs to describe the limitations
for non-vboot systems (e.g., `cros_vm` + betty), but it's nice to be up
front about what exactly this script supports.

BUG=none
TEST=`./update_kernel.sh --help`

Change-Id: Ie4dd35c27e2f855edb9e2a27d25adf044096e320
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3816710
Commit-Queue: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
1 file changed