Hi!

First off, I wanted to thank you for your great work on this tool. It makes building images of all sorts super easy. Thanks so much!

I did run into an issue where DisablePNIN is hardcoded to depend on the release version:

if manifest.release >= stretch:
            group.append(grub.DisablePNIN)

This is probably fine in most cases, but in a case where I am using systemd from jessie-backports, I can't build working ec2 images since that doesn't get called. Would it be possible to have this hinge on a config option, perhaps with a default based on release?

Thanks!