kconfig: config script: add a little user help

Give the user a clue about the problem along with the 35 lines of
usage/help text.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>