Add LwIP Ethernet example using tap network driver

Author: lschuermann

.gitmodules

@@ -10,3 +10,6 @@
 [submodule "RadioLib/RadioLib"]
 	path = RadioLib/RadioLib
 	url = https://github.com/jgromes/RadioLib.git
+[submodule "lwip/lwip"]
+	path = lwip/lwip
+	url = https://git.savannah.gnu.org/git/lwip.git

examples/lwip_ethernet_tap/Makefile

@@ -0,0 +1,20 @@
+# Makefile for LwIP TAP network example application
+
+# The default application stack and heap size allocations are not
+# nearly sufficient for LwIP. While smaller limits may well work,
+# these have been tested to work *shrug*.
+STACK_SIZE=32768
+APP_HEAP_SIZE=32768
+
+# Specify this directory relative to the current application.
+TOCK_USERLAND_BASE_DIR = ../..
+
+# Which files to compile.
+C_SRCS := $(wildcard *.c)
+
+# External libraries used
+EXTERN_LIBS += $(TOCK_USERLAND_BASE_DIR)/lwip
+
+# Include userland master makefile. Contains rules and flags for actually
+# building the application.
+include $(TOCK_USERLAND_BASE_DIR)/AppMakefile.mk