Correct SPL use of CMD_ERASEENV

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_CMD_ERASEENV defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>

include/env_internal.h

@@ -189,7 +189,7 @@ struct env_driver {
 #endif
 
 #define ENV_SAVE_PTR(x) (CONFIG_IS_ENABLED(SAVEENV) ? (x) : NULL)
-#define ENV_ERASE_PTR(x) (CONFIG_IS_ENABLED(CMD_ERASEENV) ? (x) : NULL)
+#define ENV_ERASE_PTR(x) (IS_ENABLED(CONFIG_CMD_ERASEENV) ? (x) : NULL)
 
 extern struct hsearch_data env_htab;