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>

env/flash.c

@@ -11,6 +11,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <environment.h>
 #include <linux/stddef.h>
 #include <malloc.h>