riscv: sifive: Fix OF_BOARD boot failure

riscv: sifive: Fix OF_BOARD boot failure

When using QEMU to have a quick test of booting U-Boot S-mode payload
directly without the needs of preparing the SPI flash or SD card images
for SiFive Unleashed board, as per the instructions [1], it currently
does not boot any more.

This was caused by the OF_PRIOR_STAGE removal, as gd->fdt_blob no longer
points to a valid DTB. OF_BOARD is supposed to replace OF_PRIOR_STAGE,
hence we need to add the OF_BOARD logic in board_fdt_blob_setup().

[1] https://qemu.readthedocs.io/en/latest/system/riscv/sifive_u.html#running-u-boot

Fixes: 2831fc2c4d19 ("riscv: Remove OF_PRIOR_STAGE from RISC-V boards")
Fixes: 245d83c07fb1 ("treewide: Remove OF_PRIOR_STAGE")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>