The beagle family changed how boot is configured some time ago. uboot has several boot options, the old one was to read environmental variables from flash, but for some time now uboot looks for a file boot.scr on a file system it can access. This is probably how you machine is set up.
So look in /boot - you should fine a /boot/boot.txt - that you can compile into /boot/boot.scr. Boot.txt is easier to understand than the old environmental variables, they were tree like, with one variable calling another, and so on. Boot.txt is sequential, so the command sequence is read, typical things like if an initramfs is found, then boot with that.
Anyway you can set the boot command line there (bootargs), not sure how it gets passed anymore, e.g. it may be written into the device tree - as the options passed directly onto the booting kernel are the addresses of the dev tree and initramfs ...
E.g. see:
https://archlinuxarm.org/packages/armv7h/uboot-beaglebone/files/boot.txt