efiloader: Setup FDT in autoload to fix overlays clobbering kenv

manu found in the noted PR that overlays seemed to be clobbering the kenv
and killing the boot. Further inspection revealed that one can fdt ls at
the loader prompt for a successful boot, but autoboot breaks it.

In the autoboot case, first setup of FDT is happening in the middle of
bi_load, which triggers loading of the DTBO from /boot.

This is bad, bad, bad. Files in the loader are loaded somewhere in the
middle of the address space one after another. bi_load starts building the
needed kernel bootinfo immediately after the highest-addr loaded file. File
loads in the middle of bi_load suddenly clobber bootinfo and everything goes
off the rails.

The solution to this is to use take advantage of arch_autoload to setup FDT
in efiloader compiled with LOADER_FDT_SUPPORT. This matches how it works in
ubldr land, and is how it should have worked when overlay support was added
to efiloader since fdt_setup_fdtp now has the potential to load files
(courtesy of fdt_platform_load_dtb).

PR: 230804
Discussed with: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D16858