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/imx8mn_evk_defconfig b/configs/imx8mn_evk_defconfig
index 22fad70..c18c19f 100644
--- a/configs/imx8mn_evk_defconfig
+++ b/configs/imx8mn_evk_defconfig
@@ -29,7 +29,7 @@
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_BOOTROM_SUPPORT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
-CONFIG_SPL_LEGACY_IMAGE_SUPPORT=y
+CONFIG_SPL_LEGACY_IMAGE_FORMAT=y
 CONFIG_SPL_LEGACY_IMAGE_CRC_CHECK=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x300