Document ??? feature

[nmschulte]

for the next feature ... pull from firmware/docs/rusEFI_custom_chibios.md

*) minor OS monitoring and maintainability fixes: chDbgStackOverflowPanic allows to know which thread has stack overflow ON_LOCK_HOOK is used to measure durations of critical sections

*) MMC timeout waitCounter

) Support for half/timeout-ISR for UART-DMA in /os/hal/include/uart.h and os/hal/ports/STM32/LLD/USARTv1/uart_lld. by andreika

Weird changes without an explanation:

Uncommented "ASMXOBJS" support in os/common/ports/ARMCMx/compilers/GCC/rules.mk

Use local "struct tm *t" in os/hal/ports/STM32/LLD/RTCv1/rtc_lld.c

Fixes for UART4-UART8, USART_ISR_* flags USART_CR1_M in os/hal/ports/STM32/LLD/USARTv2/serial_lld.c and os/hal/ports/STM32/LLD/USARTv2/uart_lld.c

Use QueryPerformanceCounter() instead of POSIX gettimeofday() in os/rt/ports/SIMIA32/chcore.c

New files: os/common/startup/SIMIA32/compilers/GCC/rules.mk os/hal/ports/simulator/posix/*

Also, git diff ChibiOS/stable_20.3.x d46398838ea9e0c5a85d2437cab4ccc6ad85bef9 (d463988 is current FOME choice).