moveconfig: replace unsafe eval with asteval

Commit b237d358 ("moveconfig: expand simple expressions") added
support for expanding expressions in configs, but used the unsafe python
built-in "eval". This patch fixes this by replacing eval with the
asteval module.

Signed-off-by: Markus Klotzbuecher <mk@mkio.de>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>