scripts/om-fwupgradecfg-gen.sh: Generate sha256sum for uboot verification

Future Open Mesh u-boot versions are changing the check of the image files
(vmlinux, rootfs) from md5 to sha256. Having both in them should be enough
to ensure backward and forward compatibility.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

SVN-Revision: 49140