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>

common/Makefile

@@ -238,11 +238,10 @@ obj-$(CONFIG_FIT_SIGNATURE) += image-sig.o
 obj-y += memsize.o
 obj-y += stdio.o
 
-$(obj)env_embedded.o: $(src)env_embedded.c
+$(obj)/env_embedded.o: $(src)/env_embedded.c
 	$(CC) $(AFLAGS) -Wa,--no-warn \
-		-DENV_CRC=$(shell $(obj)../tools/envcrc) \
-		-c -o $@ $(src)env_embedded.c
+		-DENV_CRC=$(shell tools/envcrc) -c -o $@ $<
 
 # SEE README.arm-unaligned-accesses
-$(obj)hush.o: CFLAGS += $(PLATFORM_NO_UNALIGNED)
-$(obj)fdt_support.o: CFLAGS += $(PLATFORM_NO_UNALIGNED)
+$(obj)/hush.o: CFLAGS += $(PLATFORM_NO_UNALIGNED)
+$(obj)/fdt_support.o: CFLAGS += $(PLATFORM_NO_UNALIGNED)