env: Fix network support when CONFIG_NET_LWIP is set

When lwIP (CONFIG_NET_LWIP) is used instead of legacy stack (CONFIG_NET),
environment flags support associated with network was not built: restore
support of "i" and "m" environment flags.

Signed-off-by: Benjamin ROBIN <dev@benjarobin.fr>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/include/env_callback.h b/include/env_callback.h
index 47a31f6..918ccb3 100644
--- a/include/env_callback.h
+++ b/include/env_callback.h
@@ -32,7 +32,7 @@
 #define DNS_CALLBACK
 #endif
 
-#ifdef CONFIG_NET
+#if CONFIG_IS_ENABLED(NET) || CONFIG_IS_ENABLED(NET_LWIP)
 #define NET_CALLBACKS \
 	"bootfile:bootfile," \
 	"ipaddr:ipaddr," \