ARM: imx6logic: Stop overwriting fdt_file if manually set

The board file uses the processor type to determine what dtb file
is set.  Unfortunately, if the user wants to manually set this,
it get gets overwritten upon boot.  This patch adds a check to
see if the value is already set and only changes it if the value
is empty.
Signed-off-by: NAdam Ford <aford173@gmail.com>

board/logicpd/imx6/imx6logic.c

@@ -152,7 +152,8 @@ int board_late_init(void)
 
 	if (is_mx6dq()) {
 		env_set("board_rev", "MX6DQ");
-		env_set("fdt_file", "imx6q-logicpd.dtb");
+		if (!env_get("fdt_file"))
+			env_set("fdt_file", "imx6q-logicpd.dtb");
 	}
 
 	return 0;