I also found fragments in p3448-0000.conf.common as well as in flash.sh but as I’m mostly using the sd card image creation method for now, I don’t understand well how to take advantage of those entry points for modifying the kernel cmdline arguments.
I think it is better using p3448-0000.conf.common to change the default kernel cmdline or try to put it in ./kernel-dts/tegra210-porg-p3448-common.dtsi → bootargs.