kbuild: add --include-dir flag only for out-of-tree build

The --include-dir flag is used to include check-in Makefiles from
$(objtree) without $(srctree)/ prefix.  Obviously, this is unneeded
for in-tree build.  Add the flag just before changing the working
directory.

This becomes effective after invoking sub-make.  Add a little bit
comments about it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>