What to write about in a master thesis that's mostly software applying common knowledge?
How do I come up with content?

The first thing to do is ask yourself why your supervisor instructed you to perform this work in the first place. Unless they wanted to give you completely meaningless work for no reason (unlikely), they are likely interested in whether the system you are building demonstrates some interesting and non-trivial result. For example, perhaps they are interested in how different types of hardware interact with each other using this library. Perhaps they think that this setup might make x protocol go more quickly. Who knows, the point is that you should find out what their motivation was, and then write your thesis as an analysis of the result they were trying to measure.

I feel like an upaid software developer disguised as a thesis student

You're not going to like this, but in my opinion, what you've described does not qualify as a master's thesis. A master's thesis is essentially a doctoral dissertation of (much) smaller scope. It is by definition a research endeavor. What you have described sounds like just a very basic software project that should take no more than a few days to hook up and measure.

I don't blame you for not knowing this, I blame your supervisor for not making this clear. Some of the main motivations for doing a master's thesis are typically to give a master's student experience with research, and to get them published so as to help them in applying to PhD programs or other research positions. In this case, I think you are justified in being confused about what content to discuss in your thesis because it doesn't sound like your supervisor has directed you to do any research whatsoever.

If you were planning on using this thesis as a boost to your career (be it for PhD applications, or in industry), it may be worth sitting down with your supervisor to figure out how to actually get you involved with research. If the supervisor is giving this kind of work to do as a thesis, then in my opinion it means that they're taking advantage of you as free labor rather than doing what they should be doing, which is guiding you as you learn how to perform research.