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>

tools/env/fw_env.c

@@ -1325,6 +1325,9 @@ static int parse_config ()
 	struct stat st;
 
 #if defined(CONFIG_FILE)
+	if (!common_args.config_file)
+		common_args.config_file = CONFIG_FILE;
+
 	/* Fills in DEVNAME(), ENVSIZE(), DEVESIZE(). Or don't. */
 	if (get_config(common_args.config_file)) {
 		fprintf(stderr, "Cannot parse config file '%s': %m\n",