Skip setting up initrd for s390x when it is not generated for the image

This fixes building minimized s390x images.

live-build/ubuntu-cpc/hooks/032-disk-image.binary

@@ -144,27 +144,32 @@ default=ubuntu
 [ubuntu]
 target = /boot
 image = /boot/vmlinuz
-ramdisk = /boot/initrd.img
 parameters = root=LABEL=cloudimg-rootfs
 EOF
 
+    ZIPL_EXTRA_PARAMS=
+    if [ -e mountpoint/boot/initrd.img-* ]; then
         # Kernel initramfs hooks end up creating a copy
         # rather than a symlink FIXME
         pushd mountpoint/boot
         ln -sf initrd.img-* initrd.img
         popd
 
+        ZIPL_EXTRA_PARAMS=--ramdisk=/boot/initrd.img
+
+        echo "ramdisk = /boot/initrd.img" >> mountpoint/etc/zipl.conf
+    fi
+
     # Create bootmap file
     chroot mountpoint /sbin/zipl -V \
         --image=/boot/vmlinuz \
-        --ramdisk=/boot/initrd.img \
         --parameters='root=LABEL=cloudimg-rootfs' \
         --target=/boot/ \
         --targetbase=$loop_device \
         --targettype=SCSI \
         --targetblocksize=512 \
-        --targetoffset=2048
-
+        --targetoffset=2048 \
+        $ZIPL_EXTRA_PARAMS
 fi
 
 if [ -n "$BOOT_MOUNTPOINT" ]; then