Correct SPL uses of CMD_FDT

This converts 2 usages of this option to the non-SPL form, since there is
no SPL_CMD_FDT defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/image-fdt.c b/boot/image-fdt.c
index b830a0a..408abce 100644
--- a/boot/image-fdt.c
+++ b/boot/image-fdt.c
@@ -272,7 +272,7 @@
 	*of_flat_tree = of_start;
 	*of_size = of_len;
 
-	if (CONFIG_IS_ENABLED(CMD_FDT))
+	if (IS_ENABLED(CONFIG_CMD_FDT))
 		set_working_fdt_addr(map_to_sysmem(*of_flat_tree));
 	return 0;