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/wandboard/wandboard.c b/board/wandboard/wandboard.c
index 74d7a17..69cdf3e 100644
--- a/board/wandboard/wandboard.c
+++ b/board/wandboard/wandboard.c
@@ -20,6 +20,7 @@
 #include <asm/mach-imx/video.h>
 #include <asm/mach-imx/sata.h>
 #include <asm/io.h>
+#include <env.h>
 #include <linux/sizes.h>
 #include <common.h>
 #include <miiphy.h>