kconfig: add build-only configurator targets

Add build-only targets for build_menuconfig, build_nconfig,
build_xconfig, and build_gconfig.
(targets must end in "config" to qualify in top-level Makefile)

This allows these target to be built without execution (e.g., to
look for errors or warnings) and/or to be built and checked by sparse.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>