Add Serial Download Protocol support as it is a useful method to
load flash.bin to RAM and run it via 'uuu'.
With this patch, it is possible to start both U-Boot SPL and U-Boot
proper using the following 'uuu'command:
$ uuu -brun spl flash.bin
Based on a patch from Marek Vasut for the imx8mm-mx8menlo board.
Also, to fit the SPL binary into the internal RAM, select CONFIG_LTO
to reduce its size.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
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