release: Support -DNO_ROOT image building

This requires a bunch of METALOG mangling to include the files we inject
into the tree. The mkisoimages.sh and make-memstick.sh scripts are now
called with the current directory inside the tree so that the relative
paths in the METALOG match up with the current directory. The scripts do
not require this when not using a METALOG, but for simplicity we always
do so. The Makefile mangles the real METALOG created from the install,
as those files are shared across all uses of the tree, but the shell
scripts create a temporary copy of the METALOG that they mangle as their
tree modifications are specific to that image. We also need to pass -D
to makefs to turn any duplicate METALOG entry errors into warnings, as
we have many (harmless) instances of those.

Whilst dvd1.iso should work, the !NOPKG code will need more work to
support this.

All media will also lack mergemaster and etcupdate trees, since more
work is needed to add -DNO_ROOT modes to them. Users of install media
built this way will have to manually bootstrap them.

Reviewed by: brooks, gjb
Differential Revision: https://reviews.freebsd.org/D33999