Data type casting added to fix warnings

Data type casting added to fix warnings

FreeRTOS before the 8.0.0 version have a xTaskCreate function
with task name parameter of type const signed char*. So the name has
to be casted to this type to avoid warnings.

There is used a macro for it, because we might want to use different
versions of the FreeRTOS, which have different data type for the name
parameter.

Look for the FREERTOS_TASK_NAME definition in os/os.h file.

This commit refs: #1021

Signed-off-by: Michal Horn <hornmich@fel.cvut.cz>