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")

This commit allows to define a device-class flag in the target
definitions. This way, it is possible to distinguish between groups
of devices in the build-process in terms of package or feature
selection.

Fixes: 071cf7b2 ("Switch to Lua for target definitions")