Draft: [CMD][FDT_OVERLAY] Add subcommand to create env vars for carrier and panel

We need to know the carrier and the panel we are running also in u-boot. As we introduced a complex logic to select overlays and don't have all needed information available in early u-boot stages, the command allow to create uboot env variables containing the compatible for the carrier and the panel. From the next boot on, these can be used in early uboot stages to configure the u-boot display output.