The the hackaday community might be a good place to train/find such people, although the are more focused on non-Linux bare-metal code I expect.

I guess you are aware of the consulting companies in this space? Baylibre and Denx (now NABLA) come to mind. Probably more Linux embedded companies on the FOSSjobs wiki. Looking at people/companies contributing to related areas of the Linux codebase is another option.

https://baylibre.com/ https://nabladev.com/ https://github.com/fossjobs/fossjobs/wiki/Resources#employme...