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 --cc Makefile
Simple merge

diff --cc arch/mips/Makefile
Simple merge

diff --cc arch/um/Makefile
Simple merge

diff --cc arch/x86/Makefile
Simple merge

diff --cc arch/x86/Makefile.um
Simple merge