Set use_nvd=0 in EC2 AMIs.

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.

Reviewed by:	gjb, imp
Relnotes:	NVMe disks in EC2 instances launched from 13.0 and later
		now show up as nda(4) devices.
Differential Revision:	https://reviews.freebsd.org/D24583

release/tools/ec2.conf

@@ -98,6 +98,10 @@ vm_extra_pre_umount() {
 	# Load the kernel module for the Amazon "Elastic Network Adapter"
 	echo 'if_ena_load="YES"' >> ${DESTDIR}/boot/loader.conf
 
+	# Use the "nda" driver for accessing NVMe disks rather than the
+	# historical "nvd" driver.
+	echo 'hw.nvme.use_nvd="0"' >> ${DESTDIR}/boot/loader.conf
+
 	# Disable ChallengeResponseAuthentication according to EC2
 	# requirements.
 	sed -i '' -e \