RTX: mbed-os-5.5 is missing mutex fix

[adamgreen]

I hit an issue today where RTX would enter an infinite loop when it attempted to release a previously acquired osMutexPrioInherit mutex.

A fix for this issue has already been made in the ARM-software/CMSIS_5 repository but doesn't appear to have made it into the release-candidate mbed-os branch yet.
ARM-software/CMSIS_5@729f6ab

[0xc0170]

cc @bulislaw

[adamgreen]

@0xc0170 let me know that 5.5 had actually released. I had missed that. I verified that this patch is still required for the 5.5 branch. https://github.com/ARMmbed/mbed-os/blob/mbed-os-5.5/rtos/rtx5/TARGET_CORTEX_M/rtx_mutex.c#L190

[bulislaw]

We haven't updated CMSIS_5 for some time now. We need to have a think how we will update CMSIS/RTX, historically we didn't do that very often. Maybe at least we can port bug fixes over.

[adamgreen]

Thanks!