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/modlwip.c

@@ -35,6 +35,8 @@
 #include "py/mperrno.h"
 #include "py/mphal.h"
 
+#if MICROPY_PY_LWIP
+
 #include "shared/netutils/netutils.h"
 
 #include "lwip/init.h"
@@ -1741,8 +1743,6 @@ STATIC mp_obj_t lwip_print_pcbs() {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(lwip_print_pcbs_obj, lwip_print_pcbs);
 
-#if MICROPY_PY_LWIP
-
 STATIC const mp_rom_map_elem_t mp_module_lwip_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_lwip) },
     { MP_ROM_QSTR(MP_QSTR_reset), MP_ROM_PTR(&mod_lwip_reset_obj) },