System memory used to boot the device in System memory boot mode.
This area is reserved for STMicroelectronics and contains the bootloader which is
used to reprogram the Flash memory through one of the following interfaces:
USART1, USART3, CAN2, USB OTG FS in Device mode (DFU: device firmware
upgrade). The bootloader is programmed by ST when the device is manufactured,
and protected against spurious write/erase operations.
这句话就证明了USART2不可以。