fdt: Rename the DEV_TREE_BIN Makefile flag to to EXT_DTB

This seems like a better name. This is a patch-up to the earlier commit
63b4b5b, and also removes a redundant Makefile change.

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$(if $(CONFIG_OF_SEPARATE),-dtb,).bin FORCE
+u-boot.img u-boot.kwb u-boot.pbl: u-boot.bin FORCE
 	$(call if_changed,mkimage)
 
 MKIMAGEFLAGS_u-boot-dtb.img = $(MKIMAGEFLAGS_u-boot.img)

doc/README.fdt-control

@@ -143,9 +143,9 @@ 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:
+EXT_DTB=<filename> to 'make', as in:
 
-	make DEV_TREE_BIN=boot/am335x-boneblack-pubkey.dtb
+	make EXT_DTB=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.

dts/Makefile

@@ -12,8 +12,8 @@ ifeq ($(DEVICE_TREE),)
 DEVICE_TREE := unset
 endif
 
-ifneq ($(DEV_TREE_BIN),)
-DTB := $(DEV_TREE_BIN)
+ifneq ($(EXT_DTB),)
+DTB := $(EXT_DTB)
 else
 DTB := arch/$(ARCH)/dts/$(DEVICE_TREE).dtb
 endif