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>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index b564294..c5f5426 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/arch/sandbox/cpu/Makefile
@@ -10,7 +10,7 @@
 obj-y	:= cpu.o os.o start.o state.o
 
 # os.c is build in the system environment, so needs standard includes
-$(obj)os.o: CFLAGS := $(filter-out -nostdinc,\
+$(obj)/os.o: CFLAGS := $(filter-out -nostdinc,\
 	$(patsubst -I%,-idirafter%,$(CFLAGS)))
-$(obj).depend.os: CPPFLAGS := $(filter-out -nostdinc,\
+$(obj)/.depend.os: CPPFLAGS := $(filter-out -nostdinc,\
 	$(patsubst -I%,-idirafter%,$(CPPFLAGS)))