Hi chipexpert,
I forgot to add that I have to use u-boot as a bootloader, which as far as I understand, doesn't allow me to use imx28_(ivt_)linux.sb file (because it's booted itself, without any external bootloader), so second method from link you provided is not for me (third also because it's about separated images).
What I found out, that there is a size limit for kernel image (if initramfs is built in then overall size), here's a topic: Discussing the Linux ARM kernel. I think that the actual cause of my problems.
Any opinion would be welcomed.
Thanks