Indeed, buildroot-s6 can be a useful example, as it will allow you to build
images for QEMU and Raspberry Pi of a Linux system where the s6 stack is used
as init system. Note that it is based on Buildroot [1], and thus is a not
fully fledged GNU/Linux distribution.
Yes. This Buildroot package provides the start up scripts as well as some
basic services. Additional services (such as running rngd) can be found in the
"overlay" subdirectories of custom/board.