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/keymile/common/common.c b/board/keymile/common/common.c
index 92d4a6f..08f7f8d 100644
--- a/board/keymile/common/common.c
+++ b/board/keymile/common/common.c
@@ -8,6 +8,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <ioports.h>
 #include <command.h>
 #include <malloc.h>