The Yocto Project.  It's not an embedded Linux Distribution,
It creates a custom one for you.

The Yocto Project (YP) is an open source collaboration project that helps developers create custom Linux-based systems for embedded products, regardless of the hardware architecture.

The project provides a flexible set of tools and a space where embedded developers worldwide can share technologies, software stacks, configurations, and best practices that can be used to create tailored Linux images for embedded and IOT devices.