boot-loader-spec: add devicetree-overlay key

boot-loader-spec: add devicetree-overlay key

Device tree overlays are a convenient way to patch device trees, e.g.,
add new devices to a device tree or enable/disable devices. This is
useful for non-discoverable but configurable hardware. Device tree
overlays are commonly used for displays on the Raspberry Pi or for
describing the content of FPGA bitstreams.

Add the devicetree-overlay key to boot loader specification entries to
allow boot loaders to apply overlays.

See #13537