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>
diff --git a/Makefile b/Makefile
index 24d9687..8436fda 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$(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)