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.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

This new build flag is mandatory for now (it may default to 0 in a future
Gluon version). It may be set to the following values:

* 0 - Do not build any images for deprecated devices.
* upgrade - Only build sysupgrade images for deprecated devices.
* full - Build both sysupgrade and factory images for deprecated devices.

"Other" images are handled like factory images, as they are also used for
the initial installation of Gluon on a device.

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