bitbake: lib/bb: support NO_COLOR

Red text on a black background can make it difficult for people with
visual impairments to read the text of error messages.  Respect the
presence of a non-empty NO_COLOR environment variable as an indication
the user doesn't want colorization to be automatically enabled.

See: https://no-color.org/
(Bitbake rev: d9986c54cd3d67ed1f7cb636b17696c8d0d4db85)

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/ui/knotty.py

@@ -179,7 +179,7 @@ class TerminalFilter(object):
             new[3] = new[3] & ~termios.ECHO
             termios.tcsetattr(fd, termios.TCSADRAIN, new)
             curses.setupterm()
-            if curses.tigetnum("colors") > 2:
+            if curses.tigetnum("colors") > 2 and os.environ.get('NO_COLOR', '') == '':
                 for h in handlers:
                     try:
                         h.formatter.enable_color()