Research ways of getting vendor BSP layers to automatically use distro_bootcmd when they are not

Taken from MEN-2404:

Many of the Vendor BSPs provide customized bootcmd logic so we need to review what is there and make sure we replicate it. That is actually easier to do with the standard U-Boot integration since we already have the MENDER_UBOOT_PRE_SETUP_COMMANDS and MENDER_UBOOT_POST_SETUP_COMMANDS hooks. Maybe it makes sense to pick one BSP and see how we can setup something similar to use GRUB-EFI?

Research ways that we can remedy this so that GRUB-EFI stays zero-conf. This ticket is mostly in the context of Yocto, but it doesn't hurt to also consider what can be done for mender-convert.