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