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/common/spl/spl_nor.c b/common/spl/spl_nor.c
index 0f4fff8..067a2d4 100644
--- a/common/spl/spl_nor.c
+++ b/common/spl/spl_nor.c
@@ -110,7 +110,7 @@
 	}
 
 	/* Legacy image handling */
-	if (IS_ENABLED(CONFIG_SPL_LEGACY_IMAGE_SUPPORT)) {
+	if (IS_ENABLED(CONFIG_SPL_LEGACY_IMAGE_FORMAT)) {
 		load.bl_len = 1;
 		load.read = spl_nor_load_read;
 		return spl_load_legacy_img(spl_image, bootdev, &load,