Hello, I am certainly not an expert but if you run the ./rploader.sh postupdate command it will spit out usage and there you can see examples of what you can specify as arguments. I believe that the postudate command would be useful only in minor updates like Update1 or Update2 where the main version number will remain the same for example 7.1.0-42661. If it was a major version number change I think a loader rebuild would be necessary.
In you case the broadwellnk-7.1.0-42661 the broadwellnk defines the platform and the 7.1.0-42661 the DSM version.
When I did the postupdate for DS918+ I had to use ./rploader.sh postupdate apollolake-7.1.0-42661. On the DS3615xs+ I used ./rploader.sh postupdate bromolow-7.1.0-42661
The codenames apollolake, bromolow etc are the Intel platform.
Try run the command ./rploader.sh postupdate only and you will see the output.
I hope I am not making any mistakes and hopefully someone with more knowledge can chime in. I did two successful postupdates with DS3615xs and DS918+ and in both cases it was intel platform-dsmversion (bromolow-7.1.0-42661, apollolake-7.1.0-42661)