esp32/mpthreadport: Use binary semaphore instead of mutex.

So a lock can be acquired on one Python thread and then released on
another.  A test for this is added.

Signed-off-by: Damien George <damien@micropython.org>

ports/esp32/mpthreadport.c

@@ -192,7 +192,10 @@ void vPortCleanUpTCB(void *tcb) {
 }
 
 void mp_thread_mutex_init(mp_thread_mutex_t *mutex) {
-    mutex->handle = xSemaphoreCreateMutexStatic(&mutex->buffer);
+    // Need a binary semaphore so a lock can be acquired on one Python thread
+    // and then released on another.
+    mutex->handle = xSemaphoreCreateBinaryStatic(&mutex->buffer);
+    xSemaphoreGive(mutex->handle);
 }
 
 int mp_thread_mutex_lock(mp_thread_mutex_t *mutex, int wait) {

tests/thread/thread_lock5.py

@@ -0,0 +1,16 @@
+# test _thread lock objects where a lock is acquired/released by a different thread
+
+import _thread
+
+
+def thread_entry():
+    print("thread about to release lock")
+    lock.release()
+
+
+lock = _thread.allocate_lock()
+lock.acquire()
+_thread.start_new_thread(thread_entry, ())
+lock.acquire()
+print("main has lock")
+lock.release()