tools: mksunxiboot: Use sunxi_image header directly

When adding eGON support to mkimage, the struct boot_file_head
definition was moved to its own header. This is the only thing
mksunxiboot needed out of asm/arch/spl.h. Clean up the relative
include by switching to new header.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>