kbuild: fix bugs in cleaning targets

"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>