py/mpconfig: Provide config option for internal printf printer.

The C-level printf is usually used for internal debugging prints, and a
port/board may want to redirect this somewhere other than stdout.

Signed-off-by: Damien George <damien@micropython.org>

py/mpconfig.h

@@ -904,6 +904,11 @@ typedef double mp_float_t;
 #define MICROPY_USE_INTERNAL_PRINTF (1)
 #endif
 
+// The mp_print_t printer used for printf output when MICROPY_USE_INTERNAL_PRINTF is enabled
+#ifndef MICROPY_INTERNAL_PRINTF_PRINTER
+#define MICROPY_INTERNAL_PRINTF_PRINTER (&mp_plat_print)
+#endif
+
 // Support for internal scheduler
 #ifndef MICROPY_ENABLE_SCHEDULER
 #define MICROPY_ENABLE_SCHEDULER (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)