init: ONFAILURE={reboot|halt|console} support

  When edeploy fail in installation process, we could have different needs.

  Values are pretty trivial to understand :
  - reboot
  - halt
  - console

  halt is the default value to keep the same behavior.

  DEBUG=1 becomes deprecated but a compatibility is kept by making an
  implicit ONFAILURE=console. This compatibility will be removed later once
  users will have migrated their configuration. At least a warning is
  shown.

  Another change of this commit is factorization of shutdown and console
  process by do_halt() and do_console() fcts.

  Enjoy,