Compilation of invalid overlays

[jimpo]

The dtc compiler allows compilation of DTS overlay files which fail when applied with FDT_ERR_NOTFOUND. I discovered this issue first with a Raspberry Pi overlay and reported the issue with instructions to reproduce in issue raspberrypi/firmware#1718.

It only fails when the overlay is compiled with --symbols as the issue appears to be with overlay symbols which have stale phandle references after the original node phandle is overwritten by the overlay.

A minimal example to reproduce is:

File base.dts:

/dts-v1/;

/ {
	soc {
		minibt: bluetooth {
			compatible = "brcm,bcm43438-bt";
			max-speed = <0x70800>;
			status = "disabled";
		};
	};
};

File blah.dts:

/dts-v1/;
/plugin/;

/ {
	fragment@0 {
		target = <&minibt>;
		minibt_frag: __overlay__ {
		};
	};
};

% dtc --symbols -I dts -O dtb overlay.dts > blah.dtbo
% dtc --symbols -I dts -O dtb base.dts > base.dtb
% fdtoverlay -v -i base.dtb -o base-with-overlay.dtb blah.dtbo
input  = base.dtb
output = base-with-overlay.dtb
overlay[0] = blah.dtbo

Failed to apply 'blah.dtbo': FDT_ERR_NOTFOUND

[dgibson]

This was part of a wider set of problems where the overlay could overwrite a base node's phandle. This should be corrected by this commit.