riscv: qemu-riscv.h: define CONFIG_PREBOOT (enables extlinux)

- Set fdt_addr variable, which is needed for extlinux to find FDT.
  Otherwise booting kernel using extlinux results in missing FDT.

- Also run fdt addr with FDT address so that fdt commands would
  work out of the box in U-Boot prompt.

This is successfully used by Fedora/RISCV with 5.1-rc3+ kernel using
OpenSBI -> U-Boot (S-mode) [extlinux] -> Kernel setup.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>