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/intel/edison/edison.c b/board/intel/edison/edison.c
index d80ee3a..f8f498d 100644
--- a/board/intel/edison/edison.c
+++ b/board/intel/edison/edison.c
@@ -4,6 +4,7 @@
  */
 #include <common.h>
 #include <dwc3-uboot.h>
+#include <env.h>
 #include <environment.h>
 #include <mmc.h>
 #include <u-boot/md5.h>