atftp: new expression for version 0.7.1

I discovered the atftpd built by the old expression failed at runtime because it needs to be linked with `libgcc_s.so`. I couldn't get the old expression to accept `NIX_LDFLAGS="-lgcc_s"`, so I wrote a new one. I also noticed that a newer version is available. I did not apply the Debian patch to this version because I don't know if that would be appropriate (Debian hasn't update to 0.7.1). atftp and atftpd run fine without the patch.

@7c6f434c I think you were the original maintainer. Are you still interested in maintaining atftp?