@igorpadykov The documentation you mentioned doesn't explain how to update rootfs in detail.
Re-generate rootfs
bitbake -c cleansstate fsl-image-af-littlekernel
bitbake fsl-image-af-littlekernel
Kernel modify, build, flash
bitbake -c compile -f linux-imx
bitbake linux-imx
bitbake -c cleansstate fsl-image-af-littlekernel
bitbake fsl-image-af-littlekernel
I think the above should be a guide.