fdt: Add DEV_TREE_BIN option to specify a device tree binary file

In some cases, an externally-built device tree binary is required to be
attached to U-Boot. An example is when using image signing, since in that
case the .dtb file must include the public keys.

Add a DEV_TREE_BIN option to the Makefile, and update the documentation.

Usage is something like:

	make DEV_TREE_BIN=boot/am335x-boneblack-pubkey.dtb

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index e77b639..866a8c4 100644
--- a/Makefile
+++ b/Makefile
@@ -832,7 +832,7 @@
 MKIMAGEFLAGS_u-boot.pbl = -n $(srctree)/$(CONFIG_SYS_FSL_PBL_RCW:"%"=%) \
 		-R $(srctree)/$(CONFIG_SYS_FSL_PBL_PBI:"%"=%) -T pblimage
 
-u-boot.img u-boot.kwb u-boot.pbl: u-boot.bin FORCE
+u-boot.img u-boot.kwb u-boot.pbl: u-boot$(if $(CONFIG_OF_SEPARATE),-dtb,).bin FORCE
 	$(call if_changed,mkimage)
 
 MKIMAGEFLAGS_u-boot-dtb.img = $(MKIMAGEFLAGS_u-boot.img)