How to correctly include system-user.dtsi into the project?

[Lavr18]

I'm trying to update the device tree by including the system-user.dtsi into the project.
I've checked your reference design meta-enclustra-refdes, but it doesn't show an example of doing it.

I've tried the way I did it in my petalinux project using the recipes-bsp/device-tree/device-tree.bbappend:

FILESEXTRAPATHS:prepend := "${THISDIR}/files:${SYSCONFIG_PATH}:"

SRC_URI:append = " file://system-user.dtsi"

and the recipes-bsp/device-tree/files/system-user.dtsi:

/include/ "system-conf.dtsi"
/ {
};

&gem3 {
	status = "okay"
};

I've intentionally made a syntax error in the .dtsi file to check if bitbake catches it when building the image. But the image builds ok and the system-user.dtsi isn't included in the final .dtb file.

What else do I need to do to include the system-user.dtsi into my project?