Introduce simple "mutex" locking code.

Locks are not normally necessary because SeaBIOS uses a cooperative
multitasking system.  However, occasionally it is necessary to be able
to lock a resource across yield calls.  This patch introduces a simple
mechanism for doing that.
diff --git a/src/stacks.c b/src/stacks.c
index a35ca3d..c783967 100644
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -248,6 +248,26 @@
         yield();
 }
 
+void
+mutex_lock(struct mutex_s *mutex)
+{
+    ASSERT32FLAT();
+    if (! CONFIG_THREADS)
+        return;
+    while (mutex->isLocked)
+        yield();
+    mutex->isLocked = 1;
+}
+
+void
+mutex_unlock(struct mutex_s *mutex)
+{
+    ASSERT32FLAT();
+    if (! CONFIG_THREADS)
+        return;
+    mutex->isLocked = 0;
+}
+
 
 /****************************************************************
  * Thread preemption
diff --git a/src/util.h b/src/util.h
index 9f71d65..9b4fd3a 100644
--- a/src/util.h
+++ b/src/util.h
@@ -208,6 +208,9 @@
 void yield(void);
 void run_thread(void (*func)(void*), void *data);
 void wait_threads(void);
+struct mutex_s { u32 isLocked; };
+void mutex_lock(struct mutex_s *mutex);
+void mutex_unlock(struct mutex_s *mutex);
 void start_preempt(void);
 void finish_preempt(void);
 void check_preempt(void);