Add "lwip" target to build open-source lwIP (via submodule).

Makefile

@@ -279,6 +279,11 @@ sdk_patch: .sdk_patch_$(VENDOR_SDK)
 empty_user_rf_pre_init.o: empty_user_rf_pre_init.c $(TOOLCHAIN)/bin/xtensa-lx106-elf-gcc
 	$(TOOLCHAIN)/bin/xtensa-lx106-elf-gcc -O2 -c $<
 
+lwip:
+	make -C esp-open-lwip -f Makefile.open install \
+	    CC=$(TOOLCHAIN)/bin/xtensa-lx106-elf-gcc \
+	    PREFIX=$(TOOLCHAIN)
+
 ESP8266_NONOS_SDK_V1.5.2_16_01_29.zip:
 	wget --content-disposition "http://bbs.espressif.com/download/file.php?id=1079"
 ESP8266_NONOS_SDK_V1.5.1_16_01_08.zip: