How to get microros thread support for freeRTOS

## Issue template

- Hardware description: generic
- RTOS: freeRTOS
- Installation type: static lib
- Version or commit hash: iron

I have generated firmware by using "ros2 run micro_ros_setup create_firmware_ws.sh generate_lib". Then I enabled the the option by using "Enable multithread support ON" in the CMakeLists.txt file. When generating the static lib I get the "#error XRCE multithreading not supported for this platform".

Is there a way to get microros thread support for platform generic running on freeRTOS?

Start with the CMakeLists.txt setting for "Enable multithread support ON" and reproduce the failure using ros2 run micro_ros_setup create_firmware_ws.sh generate_lib. Trace the reported "XRCE multithreading not supported for this platform" condition for the generic FreeRTOS platform. Done means the support question is resolved and static-library generation works with multithreading enabled, or the platform limitation is documented.

Written by the indexing model from the issue text.