extmod/modlwip: Update to work with lwIP v2.0.
extmod/modlwip: Update to work with lwIP v2.0.

lwIP v2.0.3 has been tested with this lwip module and it works very well.

pozetroninc/micropython,trezor/micropython,pozetroninc/micropython,henriknelson/micropython,bvernoux/micropython,swegener/micropython,selste/micropython,pozetroninc/micropython,kerneltask/micropython,bvernoux/micropython,dmazzella/micropython,henriknelson/micropython,pfalcon/micropython,pramasoul/micropython,adafruit/circuitpython,tobbad/micropython,tobbad/micropython,dmazzella/micropython,pfalcon/micropython,kerneltask/micropython,tralamazza/micropython,kerneltask/micropython,swegener/micropython,pramasoul/micropython,adafruit/circuitpython,trezor/micropython,adafruit/micropython,tobbad/micropython,MrSurly/micropython,tobbad/micropython,trezor/micropython,selste/micropython,trezor/micropython,swegener/micropython,adafruit/micropython,bvernoux/micropython,adafruit/micropython,adafruit/micropython,pozetroninc/micropython,adafruit/micropython,henriknelson/micropython,selste/micropython,pramasoul/micropython,pramasoul/micropython,swegener/micropython,selste/micropython,pfalcon/micropython,henriknelson/micropython,tralamazza/micropython,adafruit/circuitpython,dmazzella/micropython,kerneltask/micropython,trezor/micropython,henriknelson/micropython,tralamazza/micropython,pramasoul/micropython,swegener/micropython,dmazzella/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,selste/micropython,MrSurly/micropython,tralamazza/micropython,bvernoux/micropython,MrSurly/micropython,adafruit/circuitpython,tobbad/micropython,kerneltask/micropython,pfalcon/micropython,MrSurly/micropython,pfalcon/micropython,MrSurly/micropython,pozetroninc/micropython