Bug#840230: When installing to rootfs on LVM, an empty subvol= is appended to rootflags

[Philipp Kern <pkern@debian.org>]

Package: zipl-installer
Version: 0.0.33
Severity: serious

zipl-installer 0.0.33 breaks installation for normal non-btrfs root
filesystems.

  (initramfs) cat /proc/cmdline
  root=/dev/mapper/vg-root rootflags=subvol= BOOT_IMAGE=0 

The empty subvol= makes mount barf as it's not a valid ext4 flag. The code
does not seem too healthy to me either:

  if subvol="$(btrfs subvolume show /target 2>/dev/null | sed -n '2s/.*:[\t ]*//p')"
  then
      info "Root filesystem on btrfs subvolume ${subvol}"
      PARAMETER="$PARAMETER rootflags=subvol=${subvol}"
  fi

It'd have been more helpful to take the output and compare it to the empty
string instead.