build fixes & improvements

Fixes:

* Commit 9e2da51 renamed upgrade-debug-shell.service to
  system-upgrade-shell.service but didn't change the name in the Makefile

* Commit b7b112c refactored out read_filelist() but forgot to copy the
  declaration for GError *error from main().

Improvements:

* Generate fedup-dracut.spec from fedup-dracut.spec.in
* Add 'rpm' and 'upgrade.img' targets