py/py.mk: Update lwip build config to work with latest lwip version.
py/py.mk: Update lwip build config to work with latest lwip version.

Also, to make it possible for ports to provide their own lwipopts.h, the
default include directory of extmod/lwip-include is no longer added and
instead a port should now make sure the correct include directory is
included in the list (can still use extmod/lwip-include).

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