uboot-imx

tools: env: fix config file loading in env library

env library is broken as the config file pointer is only initialized
in main(). When running in the env library parse_config() fails:

  Cannot parse config file '(null)': Bad address

Ensure that config file pointer is always initialized.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>