doc: Fix CONFIG_OF_SEPARATE description

CONFIG_OF_SEPARATE description is not in align with actual code in Makefile and
thus has misleading instructions and explanation.

Make it aligned with the actual code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.fdt-control b/doc/README.fdt-control
index 446401c..e53cf51 100644
--- a/doc/README.fdt-control
+++ b/doc/README.fdt-control
@@ -122,13 +122,14 @@
 and development only and is not recommended for production devices.
 
 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
-a u-boot.dtb file alongside u-boot.bin. A common approach is then to
+a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
 join the two:
 
-	cat u-boot.bin u-boot.dtb >image.bin
+	cat u-boot-nodtb.bin u-boot.dtb >image.bin
 
 and then flash image.bin onto your board. Note that U-Boot creates
-u-boot-dtb.bin which does the above step for you also. If you are using
+u-boot-dtb.bin which does the above step for you also. Resulting
+u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
 CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
 tree binary.