MFC (conceptually) r328107: Add /boot/overlays

This is a direct commit to stable/11, due to path changes in HEAD. Diffs
have been applied on top of stable/11, with exception to the change to
forth/loader.conf. forth/loader.conf has seen additional changes in HEAD
mthat are not being MFC'd to stable/11, so the addition of /boot/overlays to
its module_path is not necessary.

MFC r328107: stand: Add /boot/overlays for separation of overlays from DTB

This matches directory structure used commonly in Linux-land, and it's
cleaner than mixing overlays into the existing module paths. Overlays are
still mixed in by specifying fdt_overlays in loader.conf(5).