Add a (small) banner to the Debug and Telnet CLI

Add a (small) banner to the Debug and Telnet CLI

    _______    _        _   _____  ___
 __/ __/ _ \  (_)__    | | / / _ \/ _ \
 _/ _// // / / / _ \   | |/ / ___/ ___/
 /_/ /____(_)_/\___/   |___/_/  /_/

- For terminals that look like they support ANSI output the FD.io
  part is colored red.
- This is only shown at the start of a debug CLI or a telnet CLI
  session.
- This banner can be disabled with "cli-no-banner" in the "unix"
  section of the startup config file.

Change-Id: I085b3780dcca3eae546859dbde6c1c34c8258b9f
Signed-off-by: Chris Luke <chrisy@flirble.org>