uboot-imx

tools/env: allow to pass NULL for environment options

If users of the library are happy with the default, e.g. config file
name. They can pass NULL as the opts pointer. This simplifies the
transition of existing library users.
FIXES a compile error. since common_args has been removed by
a previous patch

Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>