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>

common/image.c

@@ -189,6 +189,7 @@ static const table_entry_t uimage_type[] = {
 	{	IH_TYPE_STM32IMAGE, "stm32image", "STMicroelectronics STM32 Image" },
 	{	IH_TYPE_MTKIMAGE,   "mtk_image",   "MediaTek BootROM loadable Image" },
 	{	IH_TYPE_COPRO, "copro", "Coprocessor Image"},
+	{	IH_TYPE_SUNXI_EGON, "sunxi_egon",  "Allwinner eGON Boot Image" },
 	{	-1,		    "",		  "",			},
 };