boot-time device tree overlays with U-Boot

boot.cmd

part uuid ${devtype} ${devnum}:${bootpart} uuid
setenv bootargs console=${console} root=PARTUUID=${uuid} rw rootwait

if load ${devtype} ${devnum}:${bootpart} ${kernel_addr_r} /boot/zImage; then
  if load ${devtype} ${devnum}:${bootpart} ${fdt_addr_r} /boot/dtbs/${fdtfile}; then
	fdt addr ${fdt_addr_r}
	fdt resize

	setexpr fdtovaddr ${fdt_addr_r} + F000
	
	if load ${devtype} ${devnum}:${bootpart} ${fdtovaddr} /boot/dtbs/overlay.txt && env import -t ${fdtovaddr} ${filesize} && test -n ${overlay}; then
		echo loaded overlay.txt: ${overlay}
		for ov in ${overlay}; do
			echo overlaying ${ov}...
			load ${devtype} ${devnum}:${bootpart} ${fdtovaddr} /boot/dtbs/${ov}.dtb && fdt apply ${fdtovaddr}
		done
	fi

    if load ${devtype} ${devnum}:${bootpart} ${ramdisk_addr_r} /boot/initramfs-linux.img; then
      bootz ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r};
    else
      bootz ${kernel_addr_r} - ${fdt_addr_r};
    fi;
  fi;
fi

ina219.dts

/dts-v1/;
/plugin/;

/ {
	compatible = "allwinner,sun7i-a20";

	fragment@0 {
		target = <&i2c2>;
		__overlay__ {
			#address-cells = <1>;
			#size-cells = <0>;

			ina219@40 {
				status = "okay";
				compatible = "ti,ina219";
				reg = <0x40>;

				shunt-resistor = <100000>;	// R100
			};
		};
	};
};

overlay.txt

overlay=abc ina219