Integrate meta-seconorth-distro/fix_fng_install

Commit: seco-ne/yocto/layers/meta-seconorth-distro@620dc4db

fng-install: Fixed custom devicetree for i.MX8

A custom devicetree can be set via the -d parameter on the fng-install script. Since the re-work of the post-install scripts, the parameter wasn't applied correctly (when no overlays were set). Fixed the setting of the devicetree parameter in the boot.cfg, to re-enable to correct loading of custom devicetrees.