env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/ls2080ardb/ls2080ardb.c b/board/freescale/ls2080ardb/ls2080ardb.c
index ce419df..a278e2f 100644
--- a/board/freescale/ls2080ardb/ls2080ardb.c
+++ b/board/freescale/ls2080ardb/ls2080ardb.c
@@ -4,6 +4,7 @@
  * Copyright 2017 NXP
  */
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <errno.h>
 #include <netdev.h>