- build initramfs with sdmem support
- install kexec-tools that are used to run the sdmem-enabled initramfs on
  shutdown
- pass the rebooting/halting status to the kexec'd initramfs using a custom
  /etc/default/kexec
- remove custom live-boot packages to disable previous (buggy and incomplete)
  sdmem implementation
- provide our own tails-kexec initscript to replace /etc/init.d/kexec:
  tails-kexec is more post-eject-time friendly and informs the user s/he can
  remove the boot device before the sdmem process before it happens; hence
  switching live-boot boot parameter to noprompt
- kexec-load, tails-kexec-cache and tails-kexec are run on halt as well as on
  reboot; to achieve this we need to patch the kexec-load initscript LSB header:
  update-rc.d is not enough as insserv uses LSB headers rather than update-rc.d
  arguments
- don't disable init concurrency at shutdown anymore: the initscripts
  dependencies now are be accurate enough to prevent running in
  parallel scripts that should be run sequentially