Bug#778612: live-boot: please remove obsolete unionmount support

[Michal Suchanek <michal.suchanek@ruk.cuni.cz>]

Source: live-boot
Severity: minor

Hello,

unionmount is obsolete and requires specially patched kernel and mount
binary. It is superseded by overlayfs which is in turn superseded by
overlay.

I think there is no point keeping support for this union type and traces
of it in the live scripts can only confuse users.

Thanks

Michal

-- System Information:
Debian Release: 8.0
  APT prefers testing
  APT policy: (910, 'testing'), (900, 'stable'), (410, 'unstable'), (400, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.18.0-trunk-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8) (ignored: LC_ALL set to en_US.UTF-8)
Shell: /bin/sh linked to /bin/bash
Init: systemd (via /run/systemd/system)