Use ${terminal_output} instead of ${terminal_setup}

The newer kiwi has cleaned up its grub configurations.
One of the changes was the variable terminal_output, which you set for your terminal_output.  Much more logical.
This changes to the new way.

grub.cfg.iso-template

@@ -25,7 +25,7 @@ insmod part_gpt
 insmod ext2
 
 terminal_input console
-terminal_output ${terminal_setup}
+terminal_output ${terminal_output}
 
 set timeout=${boot_timeout}
 set timeout_style=${boot_timeout_style}