"make clean", "make clobber", "make mrproper" and "make distclean"
missed to clean-up some files when they were run with
O=<some_dir> option.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Wolfgang Denk <wd@denx.de>

This commit refactors cleaning targets such as
clean, clobber, mrpropper, distclean
with scripts/Makefile.clean.

By using scripts/Makefile.clean, we can recursively descend
into subdirectories and delete generated files there.

We do not need add a big list of generated files
to the "clean" target.

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

This commit imports build scripts from Linux Kernel v3.13
as they are.

I know they include some trailing spaces
but I am intentionally keeping them untouched.

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