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/hardware_sync/sync.c

@@ -49,6 +49,7 @@ void spin_lock_claim_mask(uint32_t mask) {
 
 void spin_lock_unclaim(uint lock_num) {
     check_lock_num(lock_num);
+    spin_unlock_unsafe(spin_lock_instance(lock_num));
     hw_claim_clear((uint8_t *) &claimed, lock_num);
 }