sunxi: Use mkimage for SPL boot image generation

Switch the SPL boot image generation from using mksunxiboot to the new
sunxi_egon format of mkimage.

Verified to create identical results for all 152 Allwinner boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Samuel Holland <samuel@sholland.org>