Added default constructor for Semaphore

Currently Semaphore can not be instantiated without an explicit count
as a constructor argument. This limits where Semaphores can be declared
and requires explicit initialization in several annoying places, such
as in member variables and SingletonPtr targets.

This adds a default count of 0, which has shown to be the most common
initial value used for semaphores.

Author: geky

rtos/rtos/Semaphore.h

@@ -31,9 +31,9 @@ namespace rtos {
 class Semaphore {
 public:
     /** Create and Initialize a Semaphore object used for managing resources.
-      @param number of available resources; maximum index value is (count-1).
+      @param number of available resources; maximum index value is (count-1). (default: 0).
     */
-    Semaphore(int32_t count);
+    Semaphore(int32_t count=0);
 
     /** Wait until a Semaphore resource becomes available.
       @param   millisec  timeout value or 0 in case of no time-out. (default: osWaitForever).