doc: riscv: flashing SiFive boards

We should not use /dev/sda and /dev/sdb in our examples. Users might
inadvertently mess up their workstation. Use /dev/sdX instead.

Remove console output like '# ' and '> ' which makes copying hard.

Set example language to bash for correct syntax-highlighting.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
iewed-by: Bin Meng <bmeng.cn@gmail.com>