Problems configuring MCUBoot to use external flash for Secondary Partition

I am using the solution for this exercise to have MCUBoot use the nrf5340dk's external flash for the secondary partition in my own project. I have done the following:

1. copied the "sysbuild" directory and "sysbuild.conf" to my project.

2. added the flash controller to my overlay:

&mx25r64 {
	status = "okay";
};

/ {
	chosen {
		nordic,pm-ext-flash = &mx25r64;
	};
};

When building using "west build -p -b nrf5340dk/nrf5340/cpuapp --sysbuild", I get the following error:

<project path>/sysbuild.conf:5: warning: attempt to assign the value 'y' to the undefined symbol PM_EXTERNAL_FLASH_MCUBOOT_SECONDARY

I am using the SDK version 2.7.0, not sure exactly what I am doing wrong. I would really appreciate some help with this.

Thanks