esp8266: Disable DEBUG_PRINTERS for 512k build.

Disabling this saves around 6000 bytes of code space and gets the 512k
build fitting in the available flash again (it increased lately due to an
increase in the size of the ESP8266 SDK).

ports/esp8266/main.c

@@ -137,3 +137,17 @@ void __assert(const char *file, int line, const char *expr) {
     for (;;) {
     }
 }
+
+#if !MICROPY_DEBUG_PRINTERS
+// With MICROPY_DEBUG_PRINTERS disabled DEBUG_printf is not defined but it
+// is still needed by esp-open-lwip for debugging output, so define it here.
+#include <stdarg.h>
+int mp_vprintf(const mp_print_t *print, const char *fmt, va_list args);
+int DEBUG_printf(const char *fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    int ret = mp_vprintf(&MICROPY_DEBUG_PRINTER_DEST, fmt, ap);
+    va_end(ap);
+    return ret;
+}
+#endif

ports/esp8266/mpconfigport_512k.h

@@ -5,6 +5,9 @@
 #undef MICROPY_EMIT_INLINE_XTENSA
 #define MICROPY_EMIT_INLINE_XTENSA      (0)
 
+#undef MICROPY_DEBUG_PRINTERS
+#define MICROPY_DEBUG_PRINTERS          (0)
+
 #undef MICROPY_ERROR_REPORTING
 #define MICROPY_ERROR_REPORTING         (MICROPY_ERROR_REPORTING_TERSE)