rockchip: mkimage: Add support for idb header V2

Rockchip BootRom supports new idb header v2 instead of legacy version.
Add support for it so that we can generate image for new SoCs.

Signed-off-by: Yi Liu <liuyi@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>