I think I know what is going on with your serial port. The STM32F4 BSP assumes that you want to use USART3 on pins PD8/PD9.
The Nucleo 446 doesn’t even have a pin PD9, being a small package part. Instead they bring out PA2 and PA3 and connect them to the virtual com port on the ST-Link processor. PA2/3 are on USART2.
So you get to define a variant. See RTEMS 7 stm32f4 console not working - #12 by kiwichris for hints.