Integrate meta-guf-distro/u-boot-imx-fw-env

Commit: https://gitlab.com/SECO-Northern-Europe/yocto/layers/meta-guf-distro/-/commit/e64bff5bbe41b38510306b5363b833ff28a1889e

fng-install-uboot: Correct naming of fw_env.config, add RAUC u-boot env variables

Fix wrong naming of fw_env.conf to fw_env.config. Add BOOT_ variables used by rauc to the list of variables to keep during an uboot update.