The output-${remote}_${revision} (or output-${revision} from the main
remote) target builds OpenWrt with the given input diffconfig,
after checking out ${revision} from ${remote}. Note that neither the
remote nor the ref or revision can have an underscore in them and
still be properly interpreted.
Still, this is all very useful -- I can kick off a from-scratch build
with `make output-pr_p2041-mpc85xx` and in a few minutes it's ready to
review.