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/remote.c

@@ -29,7 +29,7 @@ int env_init(void)
 {
 	if (crc32(0, env_ptr->data, ENV_SIZE) == env_ptr->crc) {
 		gd->env_addr = (ulong)&(env_ptr->data);
-		gd->env_valid = 1;
+		gd->env_valid = ENV_VALID;
 		return 0;
 	}