env: Add an enum for environment state

At present we have three states for the environment, numbered 0, 1 and 2.
Add an enum to record this to avoid open-coded values.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

env/ext4.c

@@ -41,7 +41,7 @@ int env_init(void)
 {
 	/* use default */
 	gd->env_addr = (ulong)&default_environment[0];
-	gd->env_valid = 1;
+	gd->env_valid = ENV_VALID;
 
 	return 0;
 }