tools: mkimage: Add Allwinner eGON support

So far we used the separate mksunxiboot tool for generating a bootable
image for Allwinner SPLs, probably just for historical reasons.

Use the mkimage framework to generate a so called eGON image the
Allwinner BROM expects.
The new image type is called "sunxi_egon", to differentiate it
from the (still to be implemented) secure boot TOC0 image.

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>