MFC r346132: stand: refactor overlay loading a little bit

It was pointed out that manually loading a .dtb to be used rather than
relying on platform-specific method for loading .dtb will result in overlays
not being applied. This was true because overlay loading was hacked into
fdt_platform_load_dtb, rather than done in a way more independent from how
the .dtb is loaded.

Instead, push overlay loading (for now) out into an
fdt_platform_load_overlays. This method easily allows ubldr to pull in any
fdt_overlays specified in the ub env, and omits overlay-checking on
platforms where they're not tested and/or not desired (e.g. powerpc). If we
eventually stop caring about fdt_overlays from ubenv (if we ever cared),
this method should get chopped out in favor of just calling
fdt_load_dtb_overlays() directly.

stand: Clean up some unintentional inconsistencies

This is a direct commit to stable/11 to address the following final
unintended inconsistencies between stable/11 and head:

- Some unused LIBSTAND= cruft left in efi/loader/Makefie
- A comment that flew in with unrelated changes
- An #include that may go away now that stand is basically self-contained