- some boards have u-boot auto-detect the board variant, and thus we unset `BOOT_FDT_FILE`
- but grub needs to know...
- so introduce a `GRUB_FDT_FILE` that is checked in case `BOOT_FDT_FILE` is not set
- this way we can keep u-boot autodetecting and grub happy
- preserve BOOT_FDT_FILE into GRUB_FDT_FILE for `nanopct6`/`nanopct6-lts`