Okay, I give up.  How do I do this under the new world order?

I have a source tree.  I want to do a build from it and install the
newly built system into a new disk mounted on, say, /mnt, such that
adding a kernel and setting up my port's boot machinery gives me a
bootable system.

How do I do this?  My best guess so far has been

% build.sh -D ... -O ... -U distribution
# build.sh -D ... -O ... -U -u -V TOOLDIR=... install=/mnt

but that leaves me with nothing but directory structure in /mnt/etc,
except for /mnt/etc/release and some files in /mnt/etc/mtree/ - and an
empty /dev, which init apparently doesn't know how to fix because
there's no /etc/MAKEDEV.

What have I done wrong?  What's the modern way to build the world and
then create an installed system from the resulting build?  This doesn't
seem like all that arcane a desire, but nothing seems to work.  I must
be missing something, but what?

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