Also the README is not clear about it, for customization you need to copy example to file named bootmenu.conf
and then edit it to your liking (i.e add your own ID and other parameters). The ID you choose can be used as root device for cal-tool or nokia flasher so it is preselected as default choice.

initfs flashing script searches for bootmenu.conf in current directory. If found it asks you if it should be added to final initfs image.