fix(devicetree):Add force flag to DTC for overlays

* Else it is not possible to  make the device tree blob when several
  variants are given. Getting a duplicate node error.

recipes-bsp/device-tree/device-tree.bbappend

@@ -3,4 +3,5 @@ FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
 SRC_URI_append = " \
     file://pl-conf.dtsi \
 "
-
+# Force for overlay
+DTC_OFLAGS_append =' -f'