extmod/modlwip: Clean up inclusion of modlwip in build process.

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>

py/py.mk

@@ -194,6 +194,7 @@ PY_EXTMOD_O_BASENAME = \
 	extmod/machine_i2c.o \
 	extmod/machine_spi.o \
 	extmod/modbluetooth.o \
+	extmod/modlwip.o \
 	extmod/modussl_axtls.o \
 	extmod/modussl_mbedtls.o \
 	extmod/moduplatform.o\