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>

extmod/extmod.cmake

@@ -16,6 +16,7 @@ set(MICROPY_SOURCE_EXTMOD
     ${MICROPY_EXTMOD_DIR}/modbluetooth.c
     ${MICROPY_EXTMOD_DIR}/modbtree.c
     ${MICROPY_EXTMOD_DIR}/modframebuf.c
+    ${MICROPY_EXTMOD_DIR}/modlwip.c
     ${MICROPY_EXTMOD_DIR}/modnetwork.c
     ${MICROPY_EXTMOD_DIR}/modonewire.c
     ${MICROPY_EXTMOD_DIR}/moduasyncio.c
@@ -246,4 +247,6 @@ if(MICROPY_PY_LWIP)
     list(APPEND MICROPY_INC_CORE
         ${MICROPY_LIB_LWIP_DIR}/include
     )
+
+    string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/lwip)
 endif()