update LwIP BACnet/IP abstraction (#246)

* update LwIP BACnet/IP abstraction. Integrate LwIP build with continuous integration.

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

ports/lwip/Makefile

@@ -0,0 +1,35 @@
+# Makefile to build BACnet/IP LwIP library
+
+# Executable file name
+TARGET = bip-lwip
+
+SRC = bip.c
+
+BACNET_DIR = ../../src
+LWIP_DIR = /usr/include/lwip
+
+CFLAGS += -I$(LWIP_DIR) -I$(BACNET_DIR)
+
+OBJS += ${SRC:.c=.o}
+
+all: Makefile $(TARGET)
+
+lib: $(TARGET)
+
+$(TARGET): $(OBJS) Makefile
+	${AR} rcs $@ $(OBJS)
+
+.c.o:
+	${CC} -c ${CFLAGS} $*.c -o $@
+
+.PHONY: depend
+depend:
+	rm -f .depend
+	${CC} -MM ${CFLAGS} *.c >> .depend
+
+.PHONY: clean
+clean:
+	-rm -f core ${TARGET} ${OBJS} $(TARGET).map
+
+.PHONY: include
+include: .depend