Fix misnamed system-upgrade-shell.service

Yet another place I forgot to change the name. Womp womp.
makefeduprepo: add './serve' script

The ./serve script uses python's SimpleHTTPServer to serve the created
repo on port 8000. Simple as heck. No guarantees about security or
sanity are made, but dang that's handy.
Makefile: add 'repo' target

With this, 'make repo' will create a valid --instrepo at ./repo.
This means you can set up a usable instrepo by doing:

    make
    sudo make install
    sudo make repo
    cd repo
    ./serve

and now you have an installable repo being served on port 8000. Handy!