As explained in commit 3204a7fb98 ("kbuild: prefix $(srctree)/ to some
included Makefiles"), I want to stop using --include-dir some day.
I already fixed up the top Makefile, but some arch Makefiles (mips, um,
x86) still include check-in Makefiles without $(srctree)/.
Fix them up so 'need-sub-make := 1' can go away for this case.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>