Set use_nvd=0 in EC2 AMIs

[cperciva]

Summary

FreeBSD is in the process of switching from nvd(4) to nda(4) as the disk device front-end to NVMe. Changing the default in the kernel is tricky since existing systems may have /dev/nvd* hard-coded e.g. in /etc/fstab; however, there's no reason to not change the default in HEAD for *new* systems.

At present I have no intention of MFCing this to stable branches, since someone might reasonably expect scripts they use for launching and configuring FreeBSD 12.1 instances to work with FreeBSD 12.2 AMIs, for example.

[cperciva]

Warner: We discussed this on IRC; please confirm that the comment in the code and what I wrote in the "summary" above make sense.

Release engineers: This is under /release/ so I figured you'd want to look at it before I commit to HEAD...