sandbox: add dummy driver ETH_SANDBOX_LWIP

Introduce ETH_SANDBOX_LWIP which enables a mock driver similar to
ETH_SANDOX but without the dependencies on the legacy network stack
(NET) so that it may be enabled when the lwIP stack (NET_LWIP) is
introduced. The driver does nothing at this stage but its presence
will allow dm_test_iommu_noiommu [1] to pass.

[1] ./u-boot -T -c "ut dm dm_test_iommu_noiommu"

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 403d7e1..88ef674 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -350,6 +350,17 @@
 
 	  This driver is particularly useful in the test/dm/eth.c tests
 
+config ETH_SANDBOX_LWIP
+	depends on SANDBOX
+	depends on NET_LWIP
+	default y
+	bool "Sandbox: Mocked Ethernet driver (for NET_LWIP)"
+	help
+	  This driver is meant as a replacement for ETH_SANDBOX when
+	  the network stack is NET_LWIP rather than NET. It currently
+	  does nothing, i.e. it drops the sent packets and never receives
+	  data.
+
 config ETH_SANDBOX_RAW
 	depends on SANDBOX
 	default y