py/mpconfig.h: Introduce MICROPY_DEBUG_PRINTER for debugging output.

This patch in effect renames MICROPY_DEBUG_PRINTER_DEST to
MICROPY_DEBUG_PRINTER, moving its default definition from
lib/utils/printf.c to py/mpconfig.h to make it official and documented, and
makes this macro a pointer rather than the actual mp_print_t struct.  This
is done to get consistency with MICROPY_ERROR_PRINTER, and provide this
macro for use outside just lib/utils/printf.c.

Ports are updated to use the new macro name.

ports/esp8266/main.c

@@ -172,7 +172,7 @@ 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);
+    int ret = mp_vprintf(MICROPY_DEBUG_PRINTER, fmt, ap);
     va_end(ap);
     return ret;
 }