lib/utils/printf: Make DEBUG_printf implementation more accessible.

The definition of DEBUG_printf doesn't depend on
MICROPY_USE_INTERNAL_PRINTF so move it out of that preprocessor
block and compile it conditionally just depending on the
MICROPY_DEBUG_PRINTERS macro. This allows a port to use DEBUG_printf
while providing it's own printf definition.

lib/utils/printf.c

@@ -26,8 +26,6 @@
 
 #include "py/mpconfig.h"
 
-#if MICROPY_USE_INTERNAL_PRINTF
-
 #include <stdint.h>
 #include <string.h>
 #include <stdarg.h>
@@ -39,6 +37,22 @@
 #include "py/formatfloat.h"
 #endif
 
+#if MICROPY_DEBUG_PRINTERS
+int DEBUG_printf(const char *fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    #ifndef MICROPY_DEBUG_PRINTER_DEST
+    #define MICROPY_DEBUG_PRINTER_DEST mp_plat_print
+    #endif
+    extern const mp_print_t MICROPY_DEBUG_PRINTER_DEST;
+    int ret = mp_vprintf(&MICROPY_DEBUG_PRINTER_DEST, fmt, ap);
+    va_end(ap);
+    return ret;
+}
+#endif
+
+#if MICROPY_USE_INTERNAL_PRINTF
+
 #undef putchar  // Some stdlibs have a #define for putchar
 int printf(const char *fmt, ...);
 int vprintf(const char *fmt, va_list ap);
@@ -59,20 +73,6 @@ int vprintf(const char *fmt, va_list ap) {
     return mp_vprintf(&mp_plat_print, fmt, ap);
 }
 
-#if MICROPY_DEBUG_PRINTERS
-int DEBUG_printf(const char *fmt, ...) {
-    va_list ap;
-    va_start(ap, fmt);
-    #ifndef MICROPY_DEBUG_PRINTER_DEST
-    #define MICROPY_DEBUG_PRINTER_DEST mp_plat_print
-    #endif
-    extern const mp_print_t MICROPY_DEBUG_PRINTER_DEST;
-    int ret = mp_vprintf(&MICROPY_DEBUG_PRINTER_DEST, fmt, ap);
-    va_end(ap);
-    return ret;
-}
-#endif
-
 // need this because gcc optimises printf("%c", c) -> putchar(c), and printf("a") -> putchar('a')
 int putchar(int c) {
     char chr = c;