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/doc/DocBook/Makefile b/doc/DocBook/Makefile
index 29b79d7..aa7c44b 100644
--- a/doc/DocBook/Makefile
+++ b/doc/DocBook/Makefile
@@ -6,8 +6,6 @@
 # To add a new book the only step required is to add the book to the
 # list of DOCBOOKS.
 
-include $(TOPDIR)/config.mk
-
 DOCBOOKS := fs.xml linker_lists.xml stdio.xml
 
 ###
@@ -122,7 +120,7 @@
 
 
 index = index.html
-main_idx = $(index)
+main_idx = doc/DocBook/$(index)
 build_main_index = rm -rf $(main_idx); \
 		   echo '<h1>U-Boot Bootloader HTML Documentation</h1>' >> $(main_idx) && \
 		   echo '<h2>U-Boot Version: $(U_BOOT_VERSION)</h2>' >> $(main_idx) && \
@@ -151,7 +149,7 @@
 	@(which xmlto > /dev/null 2>&1) || \
 	 (echo "*** You need to install xmlto ***"; \
 	  exit 1)
-	$(Q)mkdir -p $(obj)man
+	$(Q)mkdir -p $(obj)/man
 	$(call cmd_db2man)
 	@touch $@