Add /usr/lib/sysimage
    
    This came out of upstream rpm-ecosystem discussion:
    http://lists.rpm.org/pipermail/rpm-maint/2017-October/006681.html
    
    While I'm not aware of anyone starting the push to move the rpmdb
    in Fedora yet, I do plan to change rpm-ostree soon to use
    `/usr/lib/sysimage/rpm-ostree-base-db` or so for its "base" rpmdb,
    as distinct from the final one.