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>
diff --git a/board/freescale/t4rdb/spl.c b/board/freescale/t4rdb/spl.c
index 5feab1c..932954e 100644
--- a/board/freescale/t4rdb/spl.c
+++ b/board/freescale/t4rdb/spl.c
@@ -8,6 +8,7 @@
 
 #include <common.h>
 #include <console.h>
+#include <environment.h>
 #include <asm/spl.h>
 #include <malloc.h>
 #include <ns16550.h>
@@ -87,7 +88,7 @@
 			   (uchar *)CONFIG_ENV_ADDR);
 
 	gd->env_addr  = (ulong)(CONFIG_ENV_ADDR);
-	gd->env_valid = 1;
+	gd->env_valid = ENV_VALID;
 
 	i2c_init_all();