u_boot_pylib: Allow disabling colour output

When running tests there is no situation in which we want ANSI output as
it makes it much harder to see what is going on in logs, tests, etc.

Provide a way to disable this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/u_boot_pylib/tout.py b/tools/u_boot_pylib/tout.py
index 6bd2806..37849d1 100644
--- a/tools/u_boot_pylib/tout.py
+++ b/tools/u_boot_pylib/tout.py
@@ -155,7 +155,7 @@
     """
     _output(0, msg)
 
-def init(_verbose=WARNING, stdout=sys.stdout):
+def init(_verbose=WARNING, stdout=sys.stdout, allow_colour=True):
     """Initialize a new output object.
 
     Args:
@@ -166,7 +166,8 @@
 
     verbose = _verbose
     _progress = ''                    # Our last progress message
-    _color = terminal.Color()
+    _color = terminal.Color(terminal.COLOR_IF_TERMINAL if allow_colour
+                            else terminal.COLOR_NEVER)
     _stdout = stdout
 
     # TODO(sjg): Move this into Chromite libraries when we have them