env: Move env_valid to env.h

This enum is somewhat widely used to determine if the environment is valid
or not. Move it to the common environment header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/env/nowhere.c b/env/nowhere.c
index ea6c32e..7db4eec 100644
--- a/env/nowhere.c
+++ b/env/nowhere.c
@@ -9,6 +9,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <environment.h>
 #include <linux/stddef.h>