Install from the actually used staging directory.

There's still a bit of confusion with how you can change objdir,
destdir, etc. after running tools.  Perhaps we need to explicitly
specify/document the possibilities (and perhaps add a "configure"
command which changes things that can be changed without rebuilding
tools).
In a kernel-only build, build everything via sys/rump

This way, assuming you have a toolchain, you don't need anything
apart from the src/sys and src/common bits from the NetBSD tree.