When releasing a product based on the Yocto project, we have to consider that we are building on top of a multitude of different open source projects, each with different licensing requirements.
At the minimum, your embedded product will contain a bootloader (probably u-boot), the Linux kernel, and a root filesystem with one or more applications. Both u-boot and the Linux kernel are licensed under the General Public License version 2 (GPLv2), and the root filesystem could contain a variety of programs with different licenses.
All open source licenses allow you to sell a commercial product with a mixture of proprietary and open licenses as long as they are independent and the product complies with all the open source licenses. We will discuss open source and proprietary cohabiting in the Working with open source and proprietary code recipe later on.
When developing open source applications, it's important to give careful thought to the license the software will be released...