For of/devicetree using machines, set the board_type used for nvram file
selection to the first string listed in the top-level's node compatible
string, aka the machine-compatible as used by of_machine_is_compatible().

The board_type setting is used to load the board-specific nvram file with
a board-specific name so that we can ship files for each supported board
in linux-firmware.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>