This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

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.

We also make use of the boolean support of simple-uci to make scripts
clearer.

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

This commit splits gluon-config-mode into several, mostly independent
packages.

* gluon-config-mode-core: basic functionality (required)
* gluon-config-mode-hostname: hostname field
* gluon-config-mode-autoupdater: show when autoupdater is enabled
* gluon-config-mode-mesh-vpn: fastd vpn configuration, bw limit
* gluon-config-mode-geo-location: geo coordinates
* gluon-config-mode-contact-info: contact info field

The package gluon-config-mode has been removed. You need to replace
it with these packages (or any subset of them) in site.mk:

* gluon-config-mode-hostname
* gluon-config-mode-autoupdater
* gluon-config-mode-mesh-vpn
* gluon-config-mode-geo-location
* gluon-config-mode-contact-info

Note: It is not possible to deactivate the autoupdater in config mode
anymore (expert mode will still allow it). Instead, a message is shown
in case the autoupdater is enabled.