Before I answer your question, I'm going to include the following disclaimer. I am not a lawyer. I have some knowledge of business law, but my expertise is limited, and solely within the scope of the United States.
First, I'd say we need to consider looking at what the Raspberry Pi foundation states for its IP (intellectual property). In 2012 they released a blog post, Starting A Business With a Raspberry Pi. They specifically state (emphasis mine):
If, like Brian, you’re making a product which requires a Raspberry Pi to run, we don’t ask you to buy special permission or licences from us to use it. All we ask is that you include the words “Powered by Raspberry Pi” somewhere on your packaging. If your business is successful, we’d be very grateful if you could consider donating a small portion of your profits to the Raspberry Pi Foundation – but that’s all, and if you choose not to do that, that’s fine too.
Secondly, we should look at Raspbian. According to its about page, Raspbian is licensed with a standard GPL License. In a similar, vein, looking at Debian would be helpful. It is afterall, what Raspbian is based off of. They have a helpful redistribution guide, and state that it can be used commercially, and even burned to a CD and sold as is. They do however, issue a warning:
Of course, all CD manufacturers must honor the licenses of the programs in Debian. For example, many of the programs are licensed under the GPL, which requires you to distribute their source code.
For most of what comes with Raspbian, this is probably a non-issue, but you should keep this in mind for any packages you plan on installing. Each piece of software you install may have a requirement, but it should be fairly easy to discover in its licensing document. Conveniently, most open source software uses a small handful of licenses, so what's true for one application under the GPL, will usually hold for others with a GPL.
In short, I would include a mention for everything you used on an about page. It's fairly normal to list your product name, contact, copyright, and then a thank you list. The list could go something like "The Raspberry Pi Foundation, QT 5.5, package etc, etc"