nanobsd: Add a NO_ROOT build option

Add a -U option to build NanoBSD images without root privileges. It
relies on makefs/mkimg and metalog (mtree) files, similar to what
release engineering uses to build images.

Keep the current way to build NanoBSD images untouched. Once this
method gets battle tested, it may be used to build images as root as
well.

Reviewed by: imp, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48793