environment.h: fix warning: "CONFIG_ENV_IS_EMBEDDED" redefined

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
diff --git a/include/environment.h b/include/environment.h
index e8b6e45..6394a96 100644
--- a/include/environment.h
+++ b/include/environment.h
@@ -124,7 +124,9 @@
 /* The build system likes to know if the env is embedded */
 #ifdef DO_DEPS_ONLY
 # ifdef ENV_IS_EMBEDDED
-#  define CONFIG_ENV_IS_EMBEDDED
+#  ifndef CONFIG_ENV_IS_EMBEDDED
+#   define CONFIG_ENV_IS_EMBEDDED
+#  endif
 # endif
 #endif