You have to clone buildroot and this layer. When building, use the
appropriate defconfig in the buildroot-external-picoitx-imx8m/configs
directory.
git clone git://git.busybox.net/buildroot -b tbd
git clone https://github.com/kontron/buildroot-external-pitx-imx8m.git
mkdir build
cd build
make -C ../buildroot BR2_EXTERNAL=../buildroot-external-pitx-imx8m O=`pwd` kontron_pitx_imx8m_mainline_defconfig
make
The resulting bootloader, kernel, root filesystem and SD card image will
be put in the build/images directory.