fng-install:imx8: Add device tree selection and other bootscript adaptions

The TANARO board might ave different configurations and we currently have no way of expressing the variants except the different device trees Thus we want to be able to control the device tree that is being used for the TANARO.

To do this add the device tree to the possible parameters of the Flash-N-Go System and the yocto install script and evaluate the value in the gf-prepare-kernel.sh script on installation.

Additional the rootfs= in the kernel command line is set to the actually use partition.

For imx8 the devicetree files are added to DISTRO_RELEASE_ARTIFACTS so the CI will copy them to the release folder.

BCS 746-000390