Allow package.cfg on rawhide/main branch
    
    Certain packages (such as fwupd and grub2) cannot be auto-rebuilt for
    Fedora ELN due to signing requirements. They have to be built manually
    by one of the short list of people that have the correct privilege. It's
    easy to forget to build for both releases, so it would be ideal if we
    could instead allow them to carry a package.cfg file on the Rawhide
    branch that would result in `fedpkg build` building for both Rawhide and
    ELN at the same time.
    
    This patch leaves in place the check for release branches (f40, f39,
    etc.) so that the presence of the package.cfg will not have an effect
    once Rawhide is branched.
    
    Merges: https://pagure.io/fedpkg/pull-request/546
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>