Makefile: Warn against using CONFIG_SPL_FIT_GENERATOR

This option is used to run arch-specific shell scripts which produce .its
files which are used to produce FIT images. We already have binman which
is designed to produce firmware images. It is more powerful and has tests.

So this option should be deprecated and not used. Existing uses should be
migrated.

Mentions of this in code reviews over the last year or so do not seem to
have resulted in action, and things are getting worse.

So let's add a warning.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>