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/mac_eeprom.c b/board/atmel/common/mac_eeprom.c
index 7ff1ec7..f016ef7 100644
--- a/board/atmel/common/mac_eeprom.c
+++ b/board/atmel/common/mac_eeprom.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <environment.h>
 #include <i2c_eeprom.h>
 #include <netdev.h>