Add a method for blocking threads on ports.

Additionally add the first lock class since we are becoming more
concurrent.

zion/lib/mutex.cpp

@@ -0,0 +1,11 @@
+#include "lib/mutex.h"
+
+#include "debug/debug.h"
+#include "scheduler/scheduler.h"
+
+void Mutex::Lock() {
+  while (__atomic_fetch_or(&lock_, 0x1, __ATOMIC_SEQ_CST) == 0x1) {
+    dbgln("Lock sleep: %s", name_);
+    gScheduler->Preempt();
+  }
+}

zion/lib/mutex.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <stdint.h>
+
+class Mutex {
+ public:
+  Mutex(const char* name) : name_(name) {}
+
+  void Lock();
+  void Unlock() { lock_ = false; }
+
+ private:
+  const char* name_;
+
+  uint8_t lock_ = 0;
+};
+
+class MutexHolder {
+ public:
+  MutexHolder(Mutex& mutex) : mutex_(mutex) { mutex_.Lock(); }
+
+  ~MutexHolder() { mutex_.Unlock(); }
+
+  MutexHolder(MutexHolder&) = delete;
+  MutexHolder(MutexHolder&&) = delete;
+
+ private:
+  Mutex& mutex_;
+};