kbuild: improve multi-objs dependency and cleanups

Since Linux 3.18-rc1, Kbuild is able to handle multi-objs
dependency correctly, which also allows us futher cleanups
of some makefiles.

This commit imports those commits:

[1] commit c8589d1e9e01 by Masahiro Yamada
kbuild: handle multi-objs dependency appropriately

[2] commit 97e3226e6e98 by Masahiro Yamada
kbuild: handle the dependency of multi-objs hostprogs appropriately

[3] commit 022af62d0190 by Masahiro Yamada
kbuild: refactor script/kconfig/Makefile

[4] commit 221ecca6cafe by Masahiro Yamada
kbuild: remove redundant clean-files from scripts/kconfig/Makefile

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>