add TUSB_CFG_OS_TASK_PRIO to mandatory option for using an RTOS

using plain char for error enum character
increase freeRTOS configMAX_PRIORITIES to 16
house keeping & clean up compiler warning

demos/host/src/mouse_app.c

@@ -40,6 +40,8 @@
 //--------------------------------------------------------------------+
 #include "mouse_app.h"
 
+#if TUSB_CFG_HOST_HID_MOUSE
+
 //--------------------------------------------------------------------+
 // MACRO CONSTANT TYPEDEF
 //--------------------------------------------------------------------+
@@ -118,3 +120,5 @@ OSAL_TASK_FUNCTION( mouse_app_task ) (void* p_task_para)
 
   OSAL_TASK_LOOP_END
 }
+
+#endif