board: gateworks: venice: do not overwrite serial#

Do not overwrite existing serial# env to avoid:
 ## Error: Can't overwrite "serial#"
 ## Error inserting "serial#" variable, errno=1

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
diff --git a/board/gateworks/venice/imx8mm_venice.c b/board/gateworks/venice/imx8mm_venice.c
index 2a97d55..46f4bff 100644
--- a/board/gateworks/venice/imx8mm_venice.c
+++ b/board/gateworks/venice/imx8mm_venice.c
@@ -114,7 +114,8 @@
 	led_default_state();
 
 	/* Set board serial/model */
-	env_set_ulong("serial#", gsc_get_serial());
+	if (!env_get("serial#"))
+		env_set_ulong("serial#", gsc_get_serial());
 	env_set("model", gsc_get_model());
 
 	/* Set fdt_file vars */