For targets in U-Boot which were migrated to DM, the correct binary
image filename will be 'u-boot-dtb.img'. For backward compatibility,
keep support for both files and use the one which was generated with
our 'uboot-imx' package.
See also 'CONFIG_SPL_FS_LOAD_PAYLOAD_NAME' and 'CONFIG_OF_CONTROL' in
mainline U-Boot sources.
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
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