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
diff --git a/pico_sdk_init.cmake b/pico_sdk_init.cmake
index 879842c..e8405b2 100644
--- a/pico_sdk_init.cmake
+++ b/pico_sdk_init.cmake
@@ -35,8 +35,8 @@
 
     include(pico_pre_load_platform)
 
-    # todo perhaps this should be included by the platform instead?
     # We want to configure correct toolchain prior to project load
+    # todo perhaps this should be included by the platform instead?
     include(pico_pre_load_toolchain)
 
     macro(pico_sdk_init)