kbuild: use relative path more to include Makefile

Prior to this commit, it was impossible to use relative path to
include Makefiles from the top level Makefile because the option
"--include-dir=$(srctree)" becomes effective when Make enters into
sub Makefiles.

To use relative path in any places, this commit moves the option
above the "sub-make" target.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
index 95eba55..5b7e898 100644
--- a/arch/x86/Makefile.um
+++ b/arch/x86/Makefile.um
@@ -18,7 +18,7 @@
 export LDS_EXTRA
 
 # First of all, tune CFLAGS for the specific CPU. This actually sets cflags-y.
-include $(srctree)/arch/x86/Makefile_32.cpu
+include arch/x86/Makefile_32.cpu
 
 # prevent gcc from keeping the stack 16 byte aligned. Taken from i386.
 cflags-y += $(call cc-option,-mpreferred-stack-boundary=2)