Freertos path should have better correctness detection

When configuring the freertos path for a target in the Devices->MCU settings, any path is accepted without any detection.

That makes it easy to make the mistake to enter the path to the root of the FreeRTOS repo, instead of the path to the Source directory:

~/sdks/FreeRTOSv202111.00/FreeRTOS/

Instead of

~/sdks/FreeRTOSv202111.00/FreeRTOS/Source

This causes some configuration errors that are a bit hard to figure out.

A solution might be to look for portable/MemMang/heap_4.c, since that's a file that QUL requires in order for cmake configuration to succeed.