That is a good question, and one I have no answer to

I tried to use /proc/kernel_flash and it doesn't work for me. I can write the new kernel to it, if I first delete the old one and occasionally it will upgrade. Sometimes it won't. It is too unreliable for me to use. The timestamp on the file shows it has the right one, but I have no idea why it doesn't use it.

Anyway, now I have rebuilt the /dev/flash_kernel it works every time, which is nice.