kbuild: unset variables in top Makefile instead of setting 0

There is no need to set 0 to variables such as config-targets,
mixed-targets, etc.

Unset instead of setting 0 in order to use 'ifdef' to test them.

I also renamed:

  config-targets  ->  config-build
  mixed-targets   ->  mixed-build
  dot-config      ->  need-config

to clarify what we are doing.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>