add a Lock, LockGuard and printThreadInfo

* `Lock`: trivial wrapper for FreeRTOS binary semaphores
* `LockGuard`: RAII wrapper for using `Lock`
* `printThreadInfo`: helper for showing which core/FreeRTOS task we are
  running under

src/debug.h

@@ -0,0 +1,10 @@
+#pragma once
+
+namespace meshtastic
+{
+
+/// Dumps out which core we are running on, and min level of remaining stack
+/// seen.
+void printThreadInfo(const char *extra);
+
+} // namespace meshtastic