I assume the number of interrupt priorities implemented on the STM32L is the same as all the other STM32 parts, in which case just copy the configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY settings from FreeRTOS STM32 demo setting.
I don't think the idle task priority is 15 ever. It is set in the portable layer as 0. Why do you think it is 15?
Idle is always priority 0, the Tic interrupt would be 15 for that port as I believe that is one of the parts that uses big numbers for low interrupt priority.
“I don't think the idle task priority is 15 ever. It is set in the portable layer as 0. Why do you think it is 15? ”
Because on the FreeRTOS online manual it is mentioned
"on the STM32 the lowest priority you can specify in an ST driver library call is in fact 15 - and the highest priority you can specify is 0. "
The ST Driver library deals with HARDWARE INTERRUPT LEVELS, not task priorities, these are very different.
The only changes that should be needed would be to files in the portable directory tree. vTaskStartScheduler is not there. The functions that might need changing all have port at beginning of the name.