grubby-bls: also print the absolute path in the --default-kernel option

The absolute path to the kernel and initramfs images is printed by the
--info option, but it's not printed by the --default-kernel option.

Resolves: rhbz#1649778

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

grubby-bls

@@ -151,9 +151,11 @@ get_default_index() {
 }
 
 display_default_value() {
+    local prefix=$(get_prefix)
+
     case "$display_default" in
         kernel)
-            echo "${bls_linux[$default_index]}"
+            echo "${prefix}${bls_linux[$default_index]}"
             exit 0
             ;;
         index)