Create a /boot/dtb directory to house DTB blobs. The flattened device

Create a /boot/dtb directory to house DTB blobs. The flattened device
tree support includes a device tree source compiler dtc(8) which
converts .dts files into .dtb files. /boot/loader will load dtb files
from this directory by default, allowing for fewer differences between
images for different SoCs. Compiled dts files will wind up here
eventually as an alternative to embedding them into the kernel.
Document this in hier(7), as well as add missing entries for
/boot/firmware and /boot/zfs, though the latter two should only be
considered place holders if someone wants to make them better.