base-files: fix DEVICE_PRODUCT for the case when image is built for several target devices
When I build image for several target devices, /etc/device_info is generated once and contains the VERSION_PRODUCT from .config hostname is initialized using DEVICE_PRODUCT variable defined in /etc/device_info and as a result shell prompt confuses a lot.
May be it's better to fix rather in build_dir/target.../linux.../target before image generation, not sure.