Remove compiler warning by ensure prvInitialiseMutex() is not included if configUSE_M...

Remove compiler warning by ensure prvInitialiseMutex() is not included if configUSE_MUTEXES is 0.
Reduce the number of xTaskCreateStatic() parameters by having the function return the task handle, rather than pass the task handle out using a parameter.  This is also consistent with other objectCreate() functions.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2437 1d2547de-c912-0410-9cb9-b8ca96c0e9e2