fng-install: Fixed loading of overlays

Calling the append_overlay function doesn't work, because the
gf-functions script isn't copied into the fng-install script.
As a workaround, we directly write the overlays.txt file.

BCS 746-000414

recipes-bsp/fng-install/fng-install/fng-install.sh

@@ -1053,14 +1053,18 @@ load_and_write_files()
 
     # Load overlays that are provided via the script's arguments.
     # Overlays are placed in /boot/overlays and are activated by
-    # adding them to the overlays.txt (via the append_overlay
-    # function).
+    # adding them to the overlays.txt file.
     mkdir -p "${TMP_MOUNT_POINT}/overlays"
 
     for DT in ${DEVICETREEOV}; do
         DTBASE=$( basename "$DT" )
         load_file "${DT}" "${TMP_MOUNT_POINT}/overlays/${DTBASE}"
-        append_overlay "${DTBASE}" "${TMP_MOUNT_POINT}"
+
+        if [ ! -f "${TMP_MOUNT_POINT}/overlays.txt" ]; then
+            echo "overlays=" >> "${TMP_MOUNT_POINT}/overlays.txt"
+        fi
+
+        sed -i "/^overlays=/s/$/ overlays\/${DTBASE}/" "${TMP_MOUNT_POINT}/overlays.txt"
     done
 
     #================================================