fdt: Add support for a separate device tree (CONFIG_OF_SEPARATE)

This adds support for an FDT to be build as a separate binary file called
u-boot.dtb. This can be concatenated with the U-Boot binary to provide a
device tree located at run-time by U-Boot. The Makefile is modified to
provide this file in u-boot-dtb.bin.

Signed-off-by: Simon Glass <sjg@chromium.org>

README

@@ -824,8 +824,8 @@ The following options need to be configured:
 		experimental and only available on a few boards. The device
 		tree is available in the global data as gd->fdt_blob.
 
-		U-Boot needs to get its device tree from somewhere. At present
-		the only way is to embed it in the image with CONFIG_OF_EMBED.
+		U-Boot needs to get its device tree from somewhere. This can
+		be done using one of the two options below:
 
 		CONFIG_OF_EMBED
 		If this variable is defined, U-Boot will embed a device tree
@@ -834,6 +834,18 @@ The following options need to be configured:
 		is then picked up in board_init_f() and made available through
 		the global data structure as gd->blob.
 
+		CONFIG_OF_SEPARATE
+		If this variable is defined, U-Boot will build a device tree
+		binary. It will be called u-boot.dtb. Architecture-specific
+		code will locate it at run-time. Generally this works by:
+
+			cat u-boot.bin u-boot.dtb >image.bin
+
+		and in fact, U-Boot does this for you, creating a file called
+		u-boot-dtb.bin which is useful in the common case. You can
+		still use the individual files if you need something more
+		exotic.
+
 - Watchdog:
 		CONFIG_WATCHDOG
 		If this variable is defined, it enables watchdog