Flashing kernel has nothing to do with bootmenu or booting from anything. Kernel can be currently flashed only from linux PC via official flasher, not from device itself.

True that flashing kernel from device is technically possible and I have it in my TODO list. Shouldn't be hard. Just had no time and motivation. It is better to test new kernel first without flashing. And this can only be done from linux PC anyway. So when you are able to test it first (and you should) you can also flash it.