build for imx8 is internal material, maybe you can create a salesforce case, you also can refer to the imx6 guide,
2. Initialize the build environment:
$: DISTRO=nxp-imx-genivi-wayland MACHINE=imx6qpsabresd source nxp-setup-genivi.sh -b build-genivi
3. Modify the MACHINE variable from conf/local.conf to 'imx8 board'
for more detailed information, pls create a salseforce case
https://community.nxp.com/docs/DOC-329745
