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
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.