Fix bootloader not being written in Plan9

If the bootloader is not being written to a disk when installing 9front on real harware try clearing first sector of the disk with the following command.

dd if=/dev/zero of=/dev/sdX bs=512 count=1

# If command above doesn't work try this one, wait couple of seconds and
# press delete key to stop the command.
cat </dev/zero >/dev/sd*/data