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/nvram.c

@@ -104,7 +104,7 @@ int env_init(void)
 	if (crc32(0, env_ptr->data, ENV_SIZE) == env_ptr->crc) {
 		gd->env_addr	= (ulong)&env_ptr->data;
 #endif
-		gd->env_valid	= 1;
+		gd->env_valid = ENV_VALID;
 	} else {
 		gd->env_addr	= (ulong)&default_environment[0];
 		gd->env_valid	= 0;