On my k4nt, I no longer have working serial RxD, so I cannot type any commands in uboot or bist. Instead, I force commands using custom u-boot images, that I run in USB Recovery mode (USB HID).
In particular, I have a u-boot that sets fastboot mode. When I reboot into fastboot mode, I can change idme vars, flash partitions, etc.
From fastboot, I successfully flashed a linux kernel and then was able to boot to diags mode, where I can get SSH.
Unfortunately, some bug in diags or in my hardware occasionally (4 times so far) makes the next boot think it "found" a packet of idme vars that needs to be flashed. After flashing, all the idme vars are "1" again, and it cannot boot. On this k4, diags mode is not stable.
But, I was able to boot into diags mode more than once after fixing the bad idme vars.
So now I know enough about USB (HID) Recovery and fastboot to at least use both of them to get into Diags mode and get SSH, even when this k4 is pretty horribly bricked (bad idme vars AND bad linux kernel AND no serial RxD).
So, it is yielding to my will (due mostly to extreme effort). But I am learning...
I still have to find out where the dastardly "malformed idme var flash packet" is hiding and kill it, so it stops killing my idme vars.