buildroot
Buildroot is a lightweight alternative to Yocto Project.
Download a stable buildroot release from https://buildroot.org/download.html (tested with buildroot-2018.08.tar.bz2) and unpack it in a new. empty directory, say buildroot.
Goto that directory and type
make raspberrypi3_defconfig
to set the correct board type.
Check buildroot/configs for available board configs like raspberrypi0w_defconfig.
Note that we are not using the 64-Bit config raspberrypi3_64_defconfig for the Raspi3 here.
Build everything by typing:
make
This will finally create a SD card image under buildroot/output/images. Copy (dd) it to a fresh micro-SD card, put the card into the Raspi and boot (power-up).
The Raspi boots. You may login from the serial port (/dev/ttyAMA0) or use a HDMI monitor and USB keyboard.
Note that the default Raspi boot loader (bootcode.bin) is still used.
You might prefer U-Boot as a great additional boot loader.