Feature/what is network number handling (#304)

* Add What-Is-Network-Number handling.

Add What-Is-Network-Number and Network-Number-Is network layer handling.
Refactor npdu_encode_npdu_network() from router specific code.
Add unit test for NDPU network message
Add app for What-Is-Network-Number
Add app for Network-Number-Is
Add send helper for What-Is-Network-Number
Add send helper for Network-Number-Is

* added sys/debug.c to ports builds for use of debug_printf() in npdu handler.

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

apps/netnumis/Makefile

@@ -0,0 +1,40 @@
+#Makefile to build BACnet Application using GCC compiler
+
+# Executable file name
+TARGET = bacnni
+# BACnet objects that are used with this app
+BACNET_OBJECT_DIR = $(BACNET_SRC_DIR)/bacnet/basic/object
+SRC = main.c \
+	$(BACNET_OBJECT_DIR)/client/device-client.c \
+	$(BACNET_OBJECT_DIR)/netport.c
+
+# TARGET_EXT is defined in apps/Makefile as .exe or nothing
+TARGET_BIN = ${TARGET}$(TARGET_EXT)
+
+OBJS += ${SRC:.c=.o}
+
+all: ${BACNET_LIB_TARGET} Makefile ${TARGET_BIN}
+
+${TARGET_BIN}: ${OBJS} Makefile ${BACNET_LIB_TARGET}
+	${CC} ${PFLAGS} ${OBJS} ${LFLAGS} -o $@
+	size $@
+	cp $@ ../../bin
+
+${BACNET_LIB_TARGET}:
+	( cd ${BACNET_LIB_DIR} ; $(MAKE) clean ; $(MAKE) -s )
+
+.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_BIN} ${OBJS} $(TARGET).map ${BACNET_LIB_TARGET}
+
+.PHONY: include
+include: .depend
+