prepare-rtc: Fix file extension for overlays

The kernel now supports dtbo files, thus the overlays now use .dtbo as
extension. Adapt the script to select the correct files.

recipes-bsp/fng-install/prepare-image/gf-prepare-rtc.sh

@@ -27,11 +27,11 @@ cd "$BOOTFILES_DIR" || exit 1
 
 # RX8803
 if poll_rtc 0x32; then
-    append_overlay "rx8803-overlay.dtb"
+    append_overlay "rx8803-overlay.dtbo"
 else
     # PCF8563
     if poll_rtc 0x51; then
-        append_overlay "pcf8563-overlay.dtb"
+        append_overlay "pcf8563-overlay.dtbo"
     else
         echo "Error: Unable to find any valid device"
         exit 1