This allows for directives such as
makeoptions DTS+=/out/of/tree/myboard.dts
makeoptions DTS+=/out/of/tree/otherboard.dts
to be specified in config(5) and have these built/installed alongside the kernel. The assumption that overlays live in an overlays/ directory is only made for in-tree DTSO, but we still make the assumption that out-of-tree arm64 DTS will be in vendored directories (for now).
This lowers the cost to hacking on an overlay or dts by being able to quickly throw it in a custom config, especially if it doesn't fit one of the current dtb/modules quite appropriately or it's not intended for commit there.