Rework lock_core / timers (#378)

- Add recursive_mutex
  - Make all locking primitives and sleep use common overridable wait/notify support to allow RTOS
    implementations to replace WFE/SEV with something more appropriate
  - Add busy_wait_ms

src/rp2_common/pico_stdio/stdio.c

@@ -34,13 +34,15 @@ static stdio_driver_t *filter;
 auto_init_mutex(print_mutex);
 
 bool stdout_serialize_begin(void) {
-    uint core_num = get_core_num();
+    lock_owner_id_t caller = lock_get_caller_owner_id();
+    // not using lock_owner_id_t to avoid backwards incompatibility change to mutex_try_enter API
+    static_assert(sizeof(lock_owner_id_t) <= 4, "");
     uint32_t owner;
     if (!mutex_try_enter(&print_mutex, &owner)) {
-        if (owner == core_num) {
+        if (owner == (uint32_t)caller) {
             return false;
         }
-        // other core owns the mutex, so lets wait
+        // we are not a nested call, so lets wait
         mutex_enter_blocking(&print_mutex);
     }
     return true;