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>

MAKEALL

@@ -674,8 +674,6 @@ build_target() {
 		output_dir="${OUTPUT_PREFIX}"
 	fi
 
-	export BUILD_DIR="${output_dir}"
-
 	target_arch=$(get_target_arch ${target})
 	eval cross_toolchain=\$CROSS_COMPILE_`echo $target_arch | tr '[:lower:]' '[:upper:]'`
 	if [ "${cross_toolchain}" ] ; then
@@ -686,6 +684,10 @@ build_target() {
 	    MAKE=make
 	fi
 
+	if [  "${output_dir}" != "." ] ; then
+		MAKE="${MAKE} O=${output_dir}"
+	fi
+
 	${MAKE} distclean >/dev/null
 	${MAKE} -s ${target}_config