If next_node.ip4 is not set or gluon-next-node is not used, prefix4 will
now be optional as well.

and add documentation

Fixes #487

gluon-radio-config contained only a single file. The code has been adjusted
to allow creating a Gluon configuration without WLAN support by removing
the wifi24 and wifi5 sections from site.conf.

The new options allow specifying custom repositories and are evaluated
in an upgrade script, not during build.

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`.