signed kernel and initramfs on different partition than grub

[thomas-roos]

I like to place the signed kernel and initramfs files on different partition, but it seems that GRUB can not verify the signed files from there.
I tried to set root env variable. kernel is found, but this error occurs:

The file \bzImage loaded with the exit code 0xE
error: failed to verify kernel (hd0,gpt7)/bzImage
error: you need to load the kernel first