release/vm: partially support NO_ROOT

For images that don't require QEMU, support NO_ROOT. This entails:

• Passing NO_ROOT down to mk-vmimage.sh (which sets it for installworld, etc)
• Handling etcupdate bootstrapping
• Adding assorted config file METALOG entries
• Running makefs in the right directory and adding -D for dups

The main gap in basic NO_ROOT support is package installation. Each
image type must also be updated to add METALOG entries for any files it
adds.

Reviewed by: emaste
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D49320