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>