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/dts/Makefile b/dts/Makefile
index f344efe..d3122aa 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -12,8 +12,8 @@
 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