Ok, in the end it was quite easy and now i have jolla booted with my kernel. I dumped original boot partition with dd, extracted the resulting img using this help http://k.japko.eu/boot-img-manipulation.html

then rewrote zImage with my version and packed it with:

abootimg --create myboot.img -f bootimg.cfg -k zImage -r initrd.img

Booting it with

fastboot -i 0x2931 boot myboot.img

After fastboot finishes i unplug the cable so that the system boots.