MFC (conceptually) r328106: libfdt: Update to 1.4.6, use libfdt for overlays

This is a direct commit to stable/11 due to path changes in HEAD. Diffs have
been applied on top of their previous locations, with one minor adjustment
as per the "MFC" note below: a <stdlib.h> include in libfdt_env.h h as been
wrapped in an `#ifndef _STANDALONE` block to prevent build failures when
built in the bootloader context. These issues are not present in HEAD, thus
this differs from HEAD.

libfdt highlights since 1.4.3:

- fdt_property_placeholder added to create a property without specifying its
value at creation time
- stringlist helper functions added to libfdt
- Improved overlay support
- Various internal cleanup

Also switch stand/fdt over to using libfdt for overlay support with this
update. Our current overlay implementation works only for limited use cases
with overlays generated only by some specific versions of our dtc(1). Swap
it out for the libfdt implementation, which supports any properly generated
overlay being applied to a properly generated base.

This will be followed up fairly soon with an update to dtc(1) in tree to
properly generate overlays.

MFC note: the <stdlib.h> include this update introduces in libfdt_env.h is
apparently not necessary in the context we use this in. It's not immediately
clear to me the motivation for it being introduced, but it came in with
overlay support. I've left it in for the sake of accuracy and because it's
not harmful here on HEAD, but MFC'ing this to stable/11 will require
wrapping the #include in an `#ifndef _STANDALONE` block or else it will
cause build failures.