Buildroot

Buildroot emerged around 2001 among the uClibc developers, who needed an easy way to test their C library inside a complete system. It has kept that simplicity: configuration happens in the same menu interface (make menuconfig) as the Linux kernel, and the output is a ready-made disk image. New releases ship four times a year.

The philosophy is that the target has no package manager. Change something significant and you rebuild everything, which is fast enough to be reasonable. For a router, a camera or a single-purpose controller that is exactly right. Products that need several variants, updatable packages and per-file licence tracking tend to end up on the Yocto Project instead.