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)

shared/libc/printf.c

@@ -60,13 +60,13 @@ int snprintf(char *str, size_t size, const char *fmt, ...);
 int printf(const char *fmt, ...) {
     va_list ap;
     va_start(ap, fmt);
-    int ret = mp_vprintf(&mp_plat_print, fmt, ap);
+    int ret = mp_vprintf(MICROPY_INTERNAL_PRINTF_PRINTER, fmt, ap);
     va_end(ap);
     return ret;
 }
 
 int vprintf(const char *fmt, va_list ap) {
-    return mp_vprintf(&mp_plat_print, fmt, ap);
+    return mp_vprintf(MICROPY_INTERNAL_PRINTF_PRINTER, fmt, ap);
 }
 
 // need this because gcc optimises printf("%c", c) -> putchar(c), and printf("a") -> putchar('a')