esp8266: Enable MICROPY_ENABLE_FINALISER.

GC finalization should be enabled for modlwip, or it may lead to GC
problems with socket objects. This decreases usable heap size from
36288 to 35968 (-320) bytes.

esp8266/mpconfigport.h

@@ -17,6 +17,7 @@
 #define MICROPY_DEBUG_PRINTER_DEST  mp_debug_print
 #define MICROPY_READER_VFS          (MICROPY_VFS)
 #define MICROPY_ENABLE_GC           (1)
+#define MICROPY_ENABLE_FINALISER    (1)
 #define MICROPY_STACK_CHECK         (1)
 #define MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF (1)
 #define MICROPY_KBD_EXCEPTION       (1)