freertos: pristine import of FreeRTOS 7.4.2
freertos: Re-add our code for accessing the run time information
pios: changed all vPortYieldFromISR to portEND_SWITCHING_ISR
freertos stm32f1 heap1.c: readded xPortIncreaseHeapSize
pios sim: changed all vPortYieldFromISR to taskYIELD
Merge pull request #554 from lilvinz/update_freertos

WIP: Update FreeRTOS to v7.4.2