ports/sh: improve USB debugging behavior

- Set MP_DEBUG_PRINTER to go through DEBUG_printf instead of the
  console, which is important when the former is USB.
- Disable malloc/gc_alloc messages to avoid spamming USB debug log.

ports/sh/debug.c

@@ -74,6 +74,14 @@ int pe_debug_printf(char const *fmt, ...)
 int DEBUG_printf(char const *fmt, ...)
 __attribute__((alias("pe_debug_printf")));
 
+static void print_strn(void *env, const char *str, size_t len) {
+    (void)env;
+    usb_open_wait();
+    usb_fxlink_text(str, len);
+}
+
+mp_print_t const mp_debug_print = { NULL, print_strn };
+
 void pe_debug_kmalloc(void)
 {
     kmalloc_gint_stats_t *s;

ports/sh/mpconfigport.h

@@ -14,6 +14,13 @@
 /* PythonExtra's main debug flag */
 #define PE_DEBUG                          (0)
 #define MICROPY_DEBUG_VERBOSE             (0)
+/* Custom flag to remove DEBUG_printf in alloc/GC (very verbose) */
+#define MICROPY_DEBUG_VERBOSE_ALLOC       (0)
+
+#if PE_DEBUG
+extern const struct _mp_print_t mp_debug_print;
+#define MICROPY_DEBUG_PRINTER             (&mp_debug_print)
+#endif
 
 /* General feature set selection
    Other options: BASIC_FEATURES, EXTRA_FEATURES, FULL_FEATURES, EVERYTHING */

py/gc.c

@@ -34,7 +34,7 @@
 
 #if MICROPY_ENABLE_GC
 
-#if MICROPY_DEBUG_VERBOSE // print debugging info
+#if MICROPY_DEBUG_VERBOSE && MICROPY_DEBUG_VERBOSE_ALLOC
 #define DEBUG_PRINT (1)
 #define DEBUG_printf DEBUG_printf
 #else // don't print debugging info

py/malloc.c

@@ -32,7 +32,7 @@
 #include "py/misc.h"
 #include "py/mpstate.h"
 
-#if MICROPY_DEBUG_VERBOSE // print debugging info
+#if MICROPY_DEBUG_VERBOSE && MICROPY_DEBUG_VERBOSE_ALLOC
 #define DEBUG_printf DEBUG_printf
 #else // don't print debugging info
 #define DEBUG_printf(...) (void)0