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>
diff --git a/configs/kontron-sl-mx6ul_defconfig b/configs/kontron-sl-mx6ul_defconfig
index a59dacf..11e4e95 100644
--- a/configs/kontron-sl-mx6ul_defconfig
+++ b/configs/kontron-sl-mx6ul_defconfig
@@ -26,7 +26,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_BOARD_TYPES=y
 CONFIG_BOARD_EARLY_INIT_F=y
-CONFIG_SPL_LEGACY_IMAGE_SUPPORT=y
+CONFIG_SPL_LEGACY_IMAGE_FORMAT=y
 CONFIG_SPL_LEGACY_IMAGE_CRC_CHECK=y
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_USB_GADGET=y