In Bremen it has been noted that a prefix may lead people to believe it
has to stay there and thus naming their nodes "ffhb-*", which is not
what the community wants (see FreifunkBremen/gluon-site-ffhb#1).
However, an empty prefix lead to the connecting hyphen still being
inserted. This commit thus makes the hyphen part of the configured
prefix and allows the prefix to be missing from the `site.conf`.
Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.