Set rootfs for debian image

src/freedombone-prep

     shift
     MICROSD_DRIVE="$1"
     ;;
-    # BBB static IP address on the LAN
+    # debian disk image created with freedombone-image
     -i|--image)
     shift
     DEBIAN_IMAGE_FILENAME="$1"
+    ROOTFS='usb1/@'
     ;;
     # BBB static IP address on the LAN
     --ip)