kbuild: move asm-offsets.h rule to ./Kbuild ...

Currently, MIPS is the only architecture that needs
include/generated/asm-offsets.h, but we have got ./Kbuild file now.

It is a good reason to move asm-offsets.h rule from arch/mips/Makefile
to ./Kbuild and add dummy asm-offsets.c for the other architectures.
asm-offsets.h would be useful for all the architectures.

This commit does not implement include/generated/bounds.h,
but if necessary, it is easy to implement it.

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

kbuild: add ./Kbuild file to fix "make clean" ...

"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>