[ALL] Add carrier FDT overlay support

This feature allow to select a specific overlay for a carrier board. SOM can be mounted on different carrier boards compatible to their standard (Q7, SMARC, COMe...). Populating 'fdt_overlay_carrier_file' it's possible to choose the carrier overlay at runtime. If it's present, this is the first overlay to be applied. From: clea-os/bsp/nxp/u-boot-seco-imx@995b6185