build: try sha256sum if shasum is not present
build: try sha256sum if shasum is not present

shasum is not always available and sha256sum seems to exist more often
since it's part of coreutils where sha256sum comes with perl.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

rook/rook,rook/rook,rook/rook,rook/nfs,rook/rook,rook/nfs,rook/rook,rook/nfs