kbuild: move coccicheck help from scripts/Makefile.help to top Makefile

In my view, it is not helpful to have a separate file just for
the coccicheck help message.  Merge scripts/Makefile.help into
the top-level Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>