arm: mvebu: clearfog: don't override set fdtfile env

Only set the fdtfile environment variable when not already set.

Fixes: 867572f09e ("ARM: mvebu: clearfog: run-time selection of DT file")
Cc: Joel Johnson <mrjoel@lixil.net>
Reported-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Andre Heider <a.heider@gmail.com>

board/solidrun/clearfog/clearfog.c

@@ -240,6 +240,9 @@ int board_eth_init(struct bd_info *bis)
 
 int board_late_init(void)
 {
+	if (env_get("fdtfile"))
+		return 0;
+
 	cf_read_tlv_data();
 
 	if (sr_product_is(&cf_tlv_data, "Clearfog Base"))