Mainline(4.17+) overlay example.

.dtso is the device tree overlay with new style(no __overlay__, no
__fragment__ etc.).
.dts is the base device tree on with overlay will be applied.

Base dts must be compiled here because if compiled with "make dtbs",
no -@ is passed and then no __symbols__ are emitted.
Then fdt parser can't connect overlay's nodes to base nodes.

To compile:
# make

Use .dtb file with u-boot as usually
and .dtbo file apart using:
=> fdt addr 0x43000000 (dtb address)
=> fdt resize 8192 (make room for overlay)
=> fdt apply 0x43c00000 (dtbo address far enough from dtb)
=> bootz 0x42000000 - 0x43000000

Here we go!