Specify conffiles for our packages, so they aren't overwritten during
opkg updates. While this only matters during development, it is
unintended to have different behaviour for opkg update and full firmware
updates.

The package rebuild logic of OpenWrt only checks regular files. Avoid
symlinks in the files directory to make rebuilds more robust.

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.