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.

This also fixes a long-broken legacy upgrade script which would leave the
sysconfig variable lan_ifname unset and the LAN ports unusable.

gluon-config-mode retains the luci frontend, all setup scripts and config is
moved to the new package.