mx6sabre: Fix boot failure

CONFIG_SPL_LEGACY_IMAGE_SUPPORT is necessary for mx6sabresd and
mx6sabreauto, because u-boot is packed as legacy image not FIT.
However, this config is not set when CONFIG_SPL_LOAD_FIT is enabled.
Must enable it explicitly, otherwise will fail to boot like below.

U-Boot SPL 2021.04-rc2 (Feb 20 2021 - 16:43:04 -0800)
Trying to boot from MMC1
mmc_load_image_raw_sector: mmc block read error
SPL: failed to boot from all boot devices

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/configs/mx6sabreauto_defconfig b/configs/mx6sabreauto_defconfig
index 4faf49e..9ed4064 100644
--- a/configs/mx6sabreauto_defconfig
+++ b/configs/mx6sabreauto_defconfig
@@ -20,6 +20,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="imx6q-sabreauto"
 CONFIG_FIT=y
 CONFIG_SPL_FIT_PRINT=y
+CONFIG_SPL_LEGACY_IMAGE_SUPPORT=y
 CONFIG_SPL_LOAD_FIT=y
 # CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_SUPPORT_RAW_INITRD=y