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.

Reviewed by: gonzo, imp, manu
Differential Revision: https://reviews.freebsd.org/D13664