github-release.sh -- create separate checksum files

The release job for bbsim and voltctl create a composite file containing all checksums:

• https://github.com/opencord/voltctl/releases

Update logic to create one checksum per file:  voltctl-{ver}.sha256, setup being used by other github repositories creating a release.

This will make it easier for the downloader to checksum files by name rather than by manual comparison or scripting as happens now.

Oh though we should preserve the existing composite checksum file in case people/jenkins jobs are dependent on it.