In some specific cases where the device tree file is already loaded in RAM, it
can be preferable that the bootloader does not try to use a device tree from
the Linux kernel tree.
* gnu/bootloader.scm (<bootloader-configuration>)[device-tree-support?]: New field.
* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Add FDTDIR line
based on <device-tree-support?> field of <bootloader-configuration>.
* doc/guix.texi (Bootloader Configuration)[device-tree-support?]: Add
documentation for the new field.