Beaglebone Mender image doesn't enable PRUs

I’ve noticed there are no entries corresponding to PRUs of the Beaglebone Black board when booting the image with Mender.
There are some entries in /boot/uEnv.txt which configure PRUs like disable_uboot_overlay_video=1 and

###pru_rproc (4.19.x-ti kernel)

Since Mender doesn’t boot the kernel using U-Boot, I guess something should be put to boot/efi/EFI/BOOT/grub.cfg instead to load the dtbo file and disable the HDMI?

In other words, how do I enable PRUs in Mender image?

U-Boot is actually being used, but not to load the kernel. Instead U-Boot acts as a UEFI provider, where UEFI is a boot standard. The reason for doing it like this is so that we can use a standardized GRUB binary across all boards, instead of a specialized U-Boot binary.

U-Boot is the still the one which loads dtbs, so it can still be influenced. However, when using U-Boot as a UEFI provider, we execute the distro_bootcmd step, and I’m not sure if it takes uEnv.txt into account if there is a UEFI app (GRUB) present. It’s possible you will have to set this in the U-Boot environment instead, either by recompiling it or by manually modifying and saving the environment using the U-Boot prompt. Some experimentation is probably necessary here.