"make clean/mrproper/distclean" has not been working correctly.

When "make clean" is run, Kbuild descends into the directories
that are listed in "clean-dirs" variable, which includes $(srctree).

It intends descending into "./Kbuild", not "./Makefile".
(Note "Kbuild" takes precedence over "Makefile" in each directory.)

If Kbuild descends into "./Makefile", Kconfig is invoked via
"make silentoldconfig", which is not our intention.

Let's add "Kbuild" file at the top directory like Linux.

Update TODO list too.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>