Reconsider machine strategy

[lumag]

In the kirskstone branch each kind of a kit (core, industrial, vision, etc) comprised a separate machine config. While this allows flexible customizations for the kit, it results in duplicate build configurations, duplicate artifacts and resulting images. Consider taking one step back towards generic machine configurations and use single machine.conf for a family of devices. E.g. there should be a single qcs6490-rb3gen2.conf, with the 'kit' flavour being encoded into recipe names (which should only be necessary for the bootloader binaries).