fdt: Correct condition for SEPARATE_BSS

This may have different settings for SPL and TPL. Correct the condition.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 87aa677..086f0c7 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -1227,7 +1227,7 @@
 
 #ifdef CONFIG_SPL_BUILD
 	/* FDT is at end of BSS unless it is in a different memory region */
-	if (IS_ENABLED(CONFIG_SPL_SEPARATE_BSS))
+	if (CONFIG_IS_ENABLED(SEPARATE_BSS))
 		fdt_blob = (ulong *)&_image_binary_end;
 	else
 		fdt_blob = (ulong *)&__bss_end;