environment.h: fix warning: "CONFIG_ENV_IS_EMBEDDED" redefined

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>

include/environment.h

@@ -124,7 +124,9 @@ extern unsigned long nand_env_oob_offset;
 /* 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