The following changes are made:
- Guard entire file with MICROPY_PY_LWIP, so it can be included in the
build while still being disabled (for consistency with other extmod
modules).
- Add modlwip.c to list of all extmod source in py/py.mk and
extmod/extmod.cmake so all ports can easily use it.
- Move generic modlwip GIT_SUBMODULES build configuration code from
ports/rp2/CMakeLists.txt to extmod/extmod.cmake, so it can be reused by
other ports.
- Remove now unnecessary inclusion of modlwip.c in EXTMOD_SRC_C in esp8266
port, and in SRC_QSTR in mimxrt port.
Signed-off-by: Damien George <damien@micropython.org>