dtb_files cannot be used instead of dtb_file

[Fathi Boudra]

When using dtb_file, we can see a deprecation warning: use the 'dtb_files' field instead of 'dtb_file'

Unfortunately, it doesn't seems possible. dtb_file is using a hardcoded board.dtb file used to set the bootcmd.
dtb_files is only useful to copy files from rootfs to boot dir and doesn't set a default dtb file to use for bootcmd.

line 873 in boards.py:
        if self.dtb_file:
            dtb = _get_file_matching(os.path.join(path, self.dtb_file))
        if not self.dtb_file or not dtb:
            logger.warn("Could not find a valid dtb file, skipping it.")

I guess we should test dtb_files as well and set the first dtb in the list as the default.