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_runtime/runtime.c

@@ -118,7 +118,11 @@ void runtime_init(void) {
 
     // the first function pointer, not the address of it.
     for (mutex_t *m = &__mutex_array_start; m < &__mutex_array_end; m++) {
-        mutex_init(m);
+        if (m->recursion_state) {
+            recursive_mutex_init(m);
+        } else {
+            mutex_init(m);
+        }
     }
 
 #if !(PICO_NO_RAM_VECTOR_TABLE || PICO_NO_FLASH)