hw/riscv: Use pre-built bios image of generic platform for virt & sifive_u

Update virt and sifive_u machines to use the opensbi fw_dynamic bios
image built for the generic FDT platform.

Remove the out-of-date no longer used bios images.

Note:

1. To test 32-bit Linux kernel on QEMU 'sifive_u' 32-bit machine,
   the following patch is needed:
   http://lists.infradead.org/pipermail/linux-riscv/2020-July/001213.html



2. To test 64-bit Linux 5.3 kernel on QEMU 'virt' or 'sifive_u' 64-bit
   machines, the following commit should be cherry-picked to 5.3:

   commit 922b0375fc93fb1a20c5617e37c389c26bbccb70
   Author: Albert Ou <aou@eecs.berkeley.edu>
   Date:   Fri Sep 27 16:14:18 2019 -0700

       riscv: Fix memblock reservation for device tree blob

   Linux 5.4 or above already contains this commit/fix.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1596439832-29238-5-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>