moveconfig: convert ps.stderr to string

Printing the error message in verbose mode fails, since python3
doesn't implicitely convert bytes to strings.

Signed-off-by: Markus Klotzbuecher <mk@mkio.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/moveconfig.py

@@ -1217,7 +1217,7 @@ class Slot:
                                "Failed to process.\n")
         if self.options.verbose:
             self.log += color_text(self.options.color, COLOR_LIGHT_CYAN,
-                                   self.ps.stderr.read())
+                                   self.ps.stderr.read().decode())
         self.finish(False)
 
     def do_defconfig(self):