There's also Buildroot which has a bit of a learning curve (it's still easier than Yocto/Angstrom IMO) but is capable of producing highly customized, tiny embedded Linux images. At this time, there are board configs for pretty much every Rasperry Pi variant out there.
The functional equivalent of a Debian base install, including kernel, bootloader, and systemd init (approx. 350 MiB) is about 20 MiB (compressed) when built with Buildroot.
The functional equivalent of a Debian base install, including kernel, bootloader, and systemd init (approx. 350 MiB) is about 20 MiB (compressed) when built with Buildroot.