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
add attribute warn unused result for osal_create_* function
taskify keyboard_app mouse_app & led_blinking
finally able to get freeRTOS run with current mouse + keyboard example
NOTES: print_greeting if is executed before the start of freeRTOS scheduler --> hardfault
- print_greeting->vsprintf->systick -> bunch of ISR --> hardfault.

printf using serial after the start of scheduler is ok though
change OSAL_TASK_DEF to decouple variable name with task name
implement osal_task_delay for freeRTOS & non_os
getting both no_os & freertos running with mouse + keyboard