kernel-install: automatically generate "sort-key" field

Let's order by IMAGE_ID=/ID= by default.

src/kernel-install/90-loaderentry.install

@@ -62,6 +62,9 @@ fi
 
 [ -n "$PRETTY_NAME" ] || PRETTY_NAME="Linux $KERNEL_VERSION"
 
+SORT_KEY="$IMAGE_ID"
+[ -z "$SORT_KEY" ] && SORT_KEY="$ID"
+
 if [ -r /etc/kernel/cmdline ]; then
     BOOT_OPTIONS="$(tr -s "$IFS" ' ' </etc/kernel/cmdline)"
 elif [ -r /usr/lib/kernel/cmdline ]; then
@@ -130,6 +133,7 @@ mkdir -p "${LOADER_ENTRY%/*}" || {
         # See similar logic above for the systemd.machine_id= kernel command line option
         echo "machine-id $MACHINE_ID"
     fi
+    [ -n "$SORT_KEY" ] && echo "sort-key   $SORT_KEY"
     echo "options    $BOOT_OPTIONS"
     echo "linux      $ENTRY_DIR/linux"