init-install.sh: fix to handle the boot partition correctly
Previously, the boot partition was created for the target hard drive
but there was no corresponding entry for it in /etc/fstab. Besides,
even if the boot partition was mounted, it would just result in odd
directory hierarchy like /boot/boot/grub. However, what we really need
is /boot/grub. This patch fixes this problem.
Besides, for future maintance work, this patch also renames some of the
intermediate directories. It uses more descriptive names like /tgt_root
and /src_root. The name of /ssd is dropped.
