scripts: sunxi: Build an raw SPL image

Introduce a new sunxi-spl-with-ecc.bin image with already the right header,
ECC, randomizer and padding for the BROM to be able to read it.

It needs to be flashed using a raw access to the NAND so that the
controller doesn't change a thing to it, since we already have all the
right parameters.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Heiko Schocher <hs@denx.de>