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>

post/lib_powerpc/fpu/Makefile

@@ -18,7 +18,7 @@ obj-y	+= darwin-ldouble.o
 CFLAGS := $(shell echo $(CFLAGS) | sed s/-msoft-float//)
 CFLAGS += -mhard-float -fkeep-inline-functions
 
-$(addprefix $(obj),$(obj-y)): $(obj)%.o:	%.c
+$(addprefix $(obj)/,$(obj-y)): $(obj)/%.o: $(src)/%.c
 	$(CC)  $(ALL_CFLAGS) -o $@.fp $< -c
 	$(OBJCOPY) -R .gnu.attributes $@.fp $@
 	rm -f $@.fp