kbuild: change out-of-tree build

This commit changes the working directory
where the build process occurs.

Before this commit, build process occurred under the source
tree for both in-tree and out-of-tree build.

That's why we needed to add $(obj) prefix to all generated
files in makefiles like follows:
  $(obj)u-boot.bin:  $(obj)u-boot

Here, $(obj) is empty for in-tree build, whereas it points
to the output directory for out-of-tree build.

And our old build system changes the current working directory
with "make -C <sub-dir>" syntax when descending into the
sub-directories.

On the other hand, Kbuild uses a different idea
to handle out-of-tree build and directory descending.

The build process of Kbuild always occurs under the output tree.
When "O=dir/to/store/output/files" is given, the build system
changes the current working directory to that directory and
restarts the make.

Kbuild uses "make -f $(srctree)/scripts/Makefile.build obj=<sub-dir>"
syntax for descending into sub-directories.
(We can write it like "make $(obj)=<sub-dir>" with a shorthand.)
This means the current working directory is always the top
of the output directory.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Gerhard Sittig <gsi@denx.de>
diff --git a/dts/Makefile b/dts/Makefile
index 6c7198f..d81f32d 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -26,7 +26,7 @@
 # Use a constant name for this so we can access it from C code.
 # objcopy doesn't seem to allow us to set the symbol name independently of
 # the filename.
-DT_BIN	:= $(obj)dt.dtb
+DT_BIN	:= $(obj)/dt.dtb
 
 $(DT_BIN): $(TOPDIR)/board/$(VENDOR)/dts/$(DEVICE_TREE).dts
 	$(CPP) $(DTS_CPPFLAGS) $< -o $(DT_BIN).dts.tmp
@@ -38,7 +38,7 @@
 # Run the compiler and get the link script from the linker
 GET_LDS = $(CC) $(CFLAGS) $(LDFLAGS) -Wl,--verbose 2>&1
 
-$(obj)dt.o: $(DT_BIN)
+$(obj)/dt.o: $(DT_BIN)
 	# We want the output format and arch.
 	# We also hope to win a prize for ugliest Makefile / shell interaction
 	# We look in the LDSCRIPT first.
@@ -62,7 +62,7 @@
 	\
 	cd $(dir ${DT_BIN}) && \
 	$(OBJCOPY) -I binary -O $${oformat} -B $${oarch} \
-		$(notdir ${DT_BIN}) $@
+		$(notdir ${DT_BIN}) $(notdir $@)
 	rm $(DT_BIN)
 
 obj-$(CONFIG_OF_EMBED)	:= dt.o