piCore usb boot in compute module 4 (initrd inside kernel)

[mbivol10]

In https://github.com/raspberrypi/usbboot#running-your-own-not-msd-build it is said that

In order to boot usb mode the initrd has to be built into the kernel.

What steps of the source building must I do to have initrd as part of the kernel?

[mocore]

hi , mbivol10

 hint searching for : 'linux kernel config embedded initramfs' should shead some light on (initrd( aka intramfs) inside kernel)

 eg

https://wiki.gentoo.org/wiki/Custom_Initramfs#Embedding_into_the_Kernel

&

https://unix.stackexchange.com/questions/208515/difference-in-execution-of-init-with-embedded-vs-external-initramfs

... looking to make some distiction between 'initramfs initrd' in the hope it might help better answer you question !
 i found

https://unix.stackexchange.com/questions/208515/difference-in-execution-of-init-with-embedded-vs-external-initramfs

+

https://stackoverflow.com/questions/10603104/the-difference-between-initrd-and-initramfs

both seam rather insightfull , and helped refresh my understanding of things , hopfuly they also provide you with some perspective ,
wrt answering : What steps of the source building must I do to have initrd as part of the kernel?