tools: pblimage: fix image header verification function

The Layerscape platforms have different RCW header value from FSL
PowerPC platforms, the current image header verification callback
is only working on PowerPC, it will fail on Layerscape, this patch
is to fix this issue.

This is a historical problem and exposed by the following patch:
http://patchwork.ozlabs.org/project/uboot/patch/20220114173443.9877-1-pali@kernel.org



Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>