stmhal: Improve NUCLEO-F401RE build, with UART2 as REPL.

This enables MICROPY_HW_HAS_FLASH which got missed.

The HW has UART2 on the 401 connected to the STLINK procesor
which exposes it as USB serial. This connects that up so that
you can get a REPL using the USB serial.

stmhal/boards/STM32F401NUCLEO/mpconfigboard.h

@@ -2,6 +2,7 @@
 #define MICROPY_HW_MCU_NAME         "STM32F401xE"
 
 #define MICROPY_HW_HAS_SWITCH       (1)
+#define MICROPY_HW_HAS_FLASH        (1)
 #define MICROPY_HW_ENABLE_RTC       (1)
 
 // HSE is 8MHz, CPU freq set to 84MHz
@@ -16,6 +17,11 @@
 #define MICROPY_HW_UART6_PORT (GPIOC)
 #define MICROPY_HW_UART6_PINS (GPIO_PIN_6 | GPIO_PIN_7)
 
+// UART 2 connects to the STM32F103 (STLINK) on the Nucleo board
+// and this is exposed as a USB Serial port.
+#define MICROPY_HW_UART_REPL        PYB_UART_2
+#define MICROPY_HW_UART_REPL_BAUD   115200
+
 // I2C busses
 #define MICROPY_HW_I2C1_SCL (pin_B6)
 #define MICROPY_HW_I2C1_SDA (pin_B7)