usability: more informative online help

[cbrune]

The ask here is for more informative help when a user logs into the console.

When a user logs in, the contents of /etc/issue is disabled on the console. Currently this file is generated at boot time to have "mode specific" information, but it is quite basic.

The primary ask is when in installer mode that it be easy/obvious to disable the ONIE discovery process.

Along the way a couple of usability helpers are also desired. See also #684.

For example we could update /etc/issue for install mode to read:

  Please press Enter to activate this console.

  [enter is pressed here]

  ** Install Mode Enabled **
  Useful commands:
  onie-log -- check the install status
  onie-stop -- disable the image discovery process
  help -- more information

The help command (perhaps with an onie-help alias) would be a mode aware 'help' command people could run. It should contain information about the current mode and a short list of common commands useful for that mode.

That way people could run help later, after the initial /etc/issue text has scrolled away and get a few ideas.

The above output also introduces the new command onie-log -- that could simply be a wrapper for cat /var/log/onie.log.