spl: Add option to enable SPL Legacy image support

Add a Kconfig option that enables Legacy image support, this allows
boards to explicitly disable this, for instance when needed for
security reasons.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Move to common/spl/Kconfig]
Signed-off-by: Tom Rini <trini@konsulko.com>