etcupdate: Add a -N flag to perform a NO_ROOT build

This is in preparation for including an etcupdate tree when performing a
-DNO_ROOT release image build. Although -DNO_ROOT can be passed via -M,
to be useful we need to mangle the resulting METALOG to mirror the
various cleanups to the tree that are done after the build (removing
generated files, empty files and empty directories), so etcupdate needs
its own flag.

Reviewed by: jhb, pauamma
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D35857