Set default fdtfile names for unleashed and unmatched boards, as used
in the upstream Linux kernel. This allows sysboot command to find and
load appropriate dtb for the matching kernel from universal stock
Ubuntu RISC-V rootfs images based on fdtdir setting in extlinux.conf.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
cc: Paul Walmsley <paul.walmsley@sifive.com>
cc: Palmer Dabbelt <palmer@dabbelt.com>
cc: Anup Patel <anup.patel@wdc.com>
cc: Atish Patra <atish.patra@wdc.com>
cc: Pragnesh Patel <pragnesh.patel@sifive.com>
cc: Green Wan <green.wan@sifive.com>