[knc1]

Any failure of the first u-boot load and run attempt and u-boot will attempt to run the alternate system (which is diags).

From what you are working on - you probably have some sort of kernel module load error or such during start-up.

You really need some sort of fall-back plan for recovery during testing system firmware changes.
Best way is the "pro-way" - netboot the experimental stuff -
or
Second best way - load and boot from memory.

Both of which require using the serial port.
Something worth learning.

Your writing and/or changing kernel level code.
Not the same as user-land coding -
user-land code problems are well controlled, kernel level, not at all. Fatal errors there are fatal.

You never want to write to flash memory experimental code on an embedded device.
That is only a good way to shoot yourself in the foot.