patman: Update errors and warnings to use stderr

When warnings and errors are produced by tools they should be written to
stderr. Update the tout implementation to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

tools/binman/ftest.py

@@ -3232,7 +3232,7 @@ class TestFunctional(unittest.TestCase):
         with test_util.capture_sys_output() as (stdout, stderr):
             control.ReplaceEntries(updated_fname, None, outdir, [])
         self.assertIn("Skipping entry '/u-boot' from missing file",
-                      stdout.getvalue())
+                      stderr.getvalue())
 
     def testReplaceCmdMap(self):
         """Test replacing a file fron an image on the command line"""