Add Thread::join call for exiting threads without forceful termination

Allows multiple threads to join without forceful termination. Allows
threads to synchronize on cleanup.

core/mbed-rtos/rtos/Thread.cpp

@@ -39,7 +39,7 @@ Thread::Thread(void (*task)(void const *argument), void *argument,
 osStatus Thread::start(void (*task)(void const *argument), void *argument,
         osPriority priority, uint32_t stack_size, unsigned char *stack_pointer) {
     if (_tid != NULL) {
-        return osErrorResource;
+        return osErrorParameter;
     }
 
 #ifdef __MBED_CMSIS_RTOS_CM
@@ -73,6 +73,20 @@ osStatus Thread::terminate() {
     return osThreadTerminate(_tid);
 }
 
+osStatus Thread::join() {
+    while (true) {
+        uint8_t state = get_state();
+        if (state == Thread::Inactive || state == osErrorParameter) {
+            return osOK;
+        }
+
+        osStatus status = yield();
+        if (status != osOK) {
+            return status;
+        }
+    }
+}
+
 osStatus Thread::set_priority(osPriority priority) {
     return osThreadSetPriority(_tid, priority);
 }

core/mbed-rtos/rtos/Thread.h

@@ -59,6 +59,11 @@ public:
                    uint32_t stack_size=DEFAULT_STACK_SIZE,
                    unsigned char *stack_pointer=NULL);
 
+    /** Wait for thread to terminate
+      @return  status code that indicates the execution status of the function.
+    */
+    osStatus join();
+
     /** Terminate execution of a thread and remove it from Active Threads
       @return  status code that indicates the execution status of the function.
     */