stand/fdt: Consistently apply fdt_overlays

[kevans]

Summary

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.

[manu]

stand/efi/fdt/efi_fdt.c

49–50

if (hdr == NULL)
return (1);

[kevans]

Address manu@'s comment, rework flow to just return early in case of errors.