As PROVIDES can be used to replace real packages now, we don't need the
virtual packages as workaround anymore. This also means that the providing
packages don't need to be added to site.mk explicitly anymore when the
default provider is used.

Nothing outside the config mode depends on luci-base anymore.

This will require site.mk to explicitly pull in the gluon-setup-mode and
gluon-config-mode-core packages.

By setting setup_mode.skip to true in site.conf the device will skip
setup mode on first boot.

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