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/gardena/smart-gateway-at91sam/board.c b/board/gardena/smart-gateway-at91sam/board.c
index 6a1389e..3e2da0d 100644
--- a/board/gardena/smart-gateway-at91sam/board.c
+++ b/board/gardena/smart-gateway-at91sam/board.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <debug_uart.h>
+#include <env.h>
 #include <led.h>
 #include <asm/arch/at91_common.h>
 #include <asm/arch/clk.h>