@Aleksej, thanks for you investigations, I never noticed uncompressed rootfs fails and didn't test Gnu-cpio or made very big cpio, maybe topic starter will do.
Now, I have installed qemu,gnu-cpio,gnu-gzip, all I know, gzip compressed rootfs.gz fails to boot. But it works after unpacking to cpio by gzip -d
. And now, because of slow qemu, it's possible to see something new, before this day I thought initrd equals initramfs.
(gzip compressed rootfs.gz, ~30-40 lines before kernel panic)
rootfs image is not initramfs (junk in compressed archive): looks like an initrd
if unpack: (gzip -d rootfs.gz), book ok, without this message.