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>

Makefile

@@ -832,7 +832,7 @@ MKIMAGEFLAGS_u-boot.kwb = -n $(srctree)/$(CONFIG_SYS_KWD_CONFIG:"%"=%) \
 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)

doc/README.fdt-control

@@ -122,7 +122,8 @@ This should include your CPU or SOC's device tree file, placed in
 arch/<arch>/dts, and then make any adjustments required.
 
 If CONFIG_OF_EMBED is defined, then it will be picked up and built into
-the U-Boot image (including u-boot.bin).
+the U-Boot image (including u-boot.bin). This is suitable for debugging
+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
@@ -130,7 +131,10 @@ join the two:
 
 	cat u-boot.bin u-boot.dtb >image.bin
 
-and then flash image.bin onto your board.
+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
+CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
+tree binary.
 
 If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
 startup. This is only useful for sandbox. Use the -d flag to U-Boot to
@@ -138,6 +142,14 @@ specify the file to read.
 
 You cannot use more than one of these options at the same time.
 
+To use a device tree file that you have compiled yourself, pass
+DEV_TREE_BIN=<filename> to 'make', as in:
+
+	make DEV_TREE_BIN=boot/am335x-boneblack-pubkey.dtb
+
+Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
+if used, and u-boot-dtb.bin.
+
 If you wish to put the fdt at a different address in memory, you can
 define the "fdtcontroladdr" environment variable. This is the hex
 address of the fdt binary blob, and will override either of the options.

dts/Makefile

@@ -12,7 +12,11 @@ ifeq ($(DEVICE_TREE),)
 DEVICE_TREE := unset
 endif
 
+ifneq ($(DEV_TREE_BIN),)
+DTB := $(DEV_TREE_BIN)
+else
 DTB := arch/$(ARCH)/dts/$(DEVICE_TREE).dtb
+endif
 
 $(obj)/dt.dtb: $(DTB) FORCE
 	$(call if_changed,shipped)