Makefile.lib: include /__symbols__ in dtb if SPL_LOAD_FIT_APPLY_OVERLAY is enabled

In order to apply an overlay to a DTB. The DTB must have been generated
with the option '-@'.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index c10cd83..4ea898a 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -292,6 +292,10 @@
 $(obj)/%.dtb.S: $(obj)/%.dtb
 	$(call cmd,dt_S_dtb)
 
+ifeq ($(CONFIG_SPL_LOAD_FIT_APPLY_OVERLAY),y)
+DTC_FLAGS += -@
+endif
+
 quiet_cmd_dtc = DTC     $@
 # Modified for U-Boot
 # Bring in any U-Boot-specific include at the end of the file