pitx-imx8m: fix 'getSerNo' function

- Use 'board_serial' environment variable to read correct serial number
  as stored in VPD area.
Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>

board/kontron/pitx-imx8m/pitx-imx8m.c

@@ -346,7 +346,7 @@ char *getSerNo (void)
 	ENTRY e;
 	static ENTRY *ep;
 
-	e.key = "serial#";
+	e.key = "board_serial";
 	e.data = NULL;
 	hsearch_r (e, FIND, &ep, &env_htab, 0);
 	if (ep == NULL)