The precedence of different package lists was broken since #1876,
disallowing removal of GLUON_FEATURES packages via GLUON_SITE_PACKAGES.

Including all package selections, both implicit defaults and explicit
handling in Gluon, the order of precedence is now the following:

1.  OpenWrt defaults (including target-specific defaults)
2.  Device-specific packages from OpenWrt
3.  Generic default packages (from target/generic)
4.  Target default packages (target/$(GLUON_TARGET))
5.  Removal of opkg for tiny targets
6.  Packages derived from GLUON_FEATURES + GLUON_FEATURES_$(class)
7.  GLUON_SITE_PACKAGES
8.  GLUON_SITE_PACKAGES_$(class)
9.  Device-specific packages from target/$(GLUON_TARGET)
10. Device-specific packages from GLUON_$(device)_SITE_PACKAGES

This also contains various pieces of cleanup:

- No hardcoded order of device classes for target_config.lua arguments
  anymore (in fact, the Makefile doesn't know anything about device
  classes now)
- target_conifg_lib.lua only hardcodes the fallback class for x86, no
  other occurences of specific class names
- Feature -> package list mapping is moved from Makefile to the Lua code
  as well (still implemented in Shell though)

When adding device classes, targets without devices such as x86 were not
handled. As site and feature packages are included on such a per-device
decision, x86 images ended up without most packages.

Include a class setting for a target and include the class-packages
target-wide when this setting is configured.

Fixes 9c523650 ("build: introduce device classes")

It seems the CT variant was used for 11s meshing on x86 by accient. Switch
to non-CT to match the other targets.

The old bash-based parsing code was way too complex. Replace it with Lua.

Let's keep the default package selection to hardware support.

Fixes #1022

As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

This also takes us past the OpenWrt 15.05 final release.

The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.