net: lwip: add CONFIG_LWIP_DEBUG_RXTX

Add Kconfig symbol LWIP_DEBUG_RXTX to dump the incoming and outgoing
packets when NET_LWIP=y.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/net/lwip/Kconfig b/net/lwip/Kconfig
index 40345ce..d28a8a7 100644
--- a/net/lwip/Kconfig
+++ b/net/lwip/Kconfig
@@ -10,6 +10,12 @@
 	  Prints messages to the console regarding network packets that go in
           and out of the lwIP library.
 
+config LWIP_DEBUG_RXTX
+	bool "Dump packets sent and received by lwIP"
+	help
+	  Performs an hexadecimal & ASCII dump of the data received and sent by
+	  the lwIP network stack.
+
 config LWIP_ASSERT
 	bool "Enable assertions in the lwIP library"
 	help