MFC (conceptually) r327350: Consistently apply fdt_overlays

This is a direct commit to stable/11 due to restructuring of sys/boot =>
stand in -HEAD. The diff remains the same and it is simply applied to the
previous location.

MFC r327350: stand/fdt: Consistently apply fdt_overlays

Overlays were previously not applied when U-Boot provides FDT or EFI
provides FDT, only when we load FDT from /boot/dtb given name from U-Boot.

Make all three paths lead to loading fdt_overlays and applying them, so that
fdt_overlays can be expected to Just Work.