Add support for expanding simple expressions and sizes such as
"(4 * 1024)", "(512 << 10)" or "(SZ_256K)".

This can help to significantly reduce the number of "suspicious"
moves, such as

 'CONFIG_ENV_SIZE="(64 << 10)"' was removed by savedefconfig.

If the expansion fails, it falls back to the original string.

Signed-off-by: Markus Klotzbuecher <markus.klotzbuecher@kistler.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Heiko Schocher <hs@denx.de>