rockchip: mkimage: Remove host endianness dependency

The Rockchip boot ROM expects little-endian values in the image header.
When running mkimage on a big-endian machine, these values need to be
byteswapped before writing or verifying the header.

This change fixes cross-compiling U-Boot SPL for the RK3399 SoC from a
big-endian ppc64 host machine.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>