AFAIK, update cmdline.txt
At least on the initial builds of Core on Rpi, before I handed it off to Bmarkus, on the sdcard, there is/was a file named cmdline.txt
You would have to edit this plain text file to add the nodhcp boot code, or any other boot codes.
On Armv7 systems I had made a script to mount the sdcard and call an editor for such purposes. I don't know of Bmarkus has made such available.
Anyway, I would start by checking if sdcard is mounted, if not mount it and look for cmdline.txt. Be sure to use a text editor to add nodhcp to the end of the line and do not start a new line.
I am sure Bmarkus will chime in if the procedure has changed from my initial builds.