Makefile: use two double-quotations as a pair

Some editors such as Emacs can highlight source files.
But their parser algorithm is not perfect.

If you use one double-quotation alone, some editor cannot
handle it nicely and mark source lines as a string by mistake.

It is preferable to use two double-quotations as a pair.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/dts/Makefile b/dts/Makefile
index 140c8bc..6c7198f 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -10,7 +10,7 @@
 ifeq ($(DEVICE_TREE),)
 $(if $(CONFIG_DEFAULT_DEVICE_TREE),,\
 $(error Please define CONFIG_DEFAULT_DEVICE_TREE in your board header file))
-DEVICE_TREE = $(subst ",,$(CONFIG_DEFAULT_DEVICE_TREE))
+DEVICE_TREE = $(CONFIG_DEFAULT_DEVICE_TREE:"%"=%)
 endif
 
 DTS_INCDIRS =  $(SRCTREE)/board/$(VENDOR)/$(BOARD)/dts