spl: Rename Kconfig SPL_LEGACY_IMAGE_SUPPORT to SPL_LEGACY_IMAGE_FORMAT

This matches what this support is called in the non-SPL case. The postfix
_SUPPORT is redundant as enabling Kconfig options implies support.
With this we can use CONFIG_IS_ENABLED() as needed.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>