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/atmel/common/board.c b/board/atmel/common/board.c
index 20c3b8c..c41706c 100644
--- a/board/atmel/common/board.c
+++ b/board/atmel/common/board.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <w1.h>
 #include <w1-eeprom.h>
 #include <dm/device-internal.h>