Checksums of build artefacts

[Till Rohrmann]

When checking the current release candidat I struggled to verify the md5
and sha512 checksum using md5sum and shasum. The reason is that the
generated md5 and sha files are not compliant to the format md5sum and
shasum require. The checksums are correct, though. In order to avoid manual
checking, it would be helpful to change the format.