tweaking kernel boot argument

[WombleGoneBad]

Hi,
I need to change the bootargs permanently for my samsung chromebook system (in my case because I need to use change the init from systemd to a custom script). I have done this before, but it was a while ago, and I'm not really familiar with uboot, and I'm having real trouble finding an instruction on how to do this correctly.
I *think* i need to change a some 'boot.cmd' file and use it and recreate the boot.scr.img, but i cant find any boot.cmd file.
Can someone point me in the right direction? I feel a bit silly for asking, as i'm sure there is an instruction somewhere for this, but if there is i really cant find it!
thanks.

[WarheadsSE]

Extract it from the existing script, but removing the header.