yast2-product-creator

I’m trying to create a own product based on SLE-12-Server (yast2-product-creator-3.1.5-1.204.noarch)
It failed with the following error:

Error
Could not ad GPG key XXXXXXXX to initrd
boot/x86_64/loader/initrd

So far I see this is because now xz insted gzip for initrd compression is used.
If is modify the ProductCreator.rn fil ethis way I get a bootable iso.

But here the boot failes with the following error.

List of all partitions:
No filesystem could mount root, tried:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Does anyone have an idea?
Maybe the kernel itsself or its module with the inird have to be signed as well with the own key???

Thanks,
Birger

An addendum by myself:

When ever I change the initrd the iso boot will not be able to mount the squashed rootfs (mybe it cannot load the kernel module) .
So I keep the initrd untouched.
In order to prevent linuxrc to fail by invalid signature I add the boot parameter insecure=1.

In this way I’m able to build an customized SLE-12-Server ISO.