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 9c52365077 ("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.
