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/cmd/net.c b/cmd/net.c
index 89721b8..2374039 100644
--- a/cmd/net.c
+++ b/cmd/net.c
@@ -9,6 +9,7 @@
  */
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <net.h>
 
 static int netboot_common(enum proto_t, cmd_tbl_t *, int, char * const []);