kernel-binary: Do not include sourcedir in certificate path.
    
    The certs macro runs before build directory is set up so it creates the
    aggregate of supplied certificates in the source directory.
    
    Using this file directly as the certificate in kernel config works but
    embeds the source directory path in the kernel config.
    
    To avoid this symlink the certificate to the build directory and use
    relative path to refer to it.
    
    Also fabricate a certificate in the same location in build directory
    when none is provided.