py: Add verbose debug compile-time flag MICROPY_DEBUG_VERBOSE.

It enables all the DEBUG_printf outputs in the py/ source code.
diff --git a/py/gc.c b/py/gc.c
index 7253b7d..3a505e9 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -35,7 +35,7 @@
 
 #if MICROPY_ENABLE_GC
 
-#if 0 // print debugging info
+#if MICROPY_DEBUG_VERBOSE // print debugging info
 #define DEBUG_PRINT (1)
 #define DEBUG_printf DEBUG_printf
 #else // don't print debugging info