add -Wextra for more warnings to example

non-stack warning (probably mcu driver) should be suppressed in the
board.mk

hw/bsp/stm32h743nucleo/stm32h743nucleo.c

@@ -197,11 +197,13 @@ uint32_t board_button_read(void)
 
 int board_uart_read(uint8_t* buf, int len)
 {
+  (void) buf; (void) len;
   return 0;
 }
 
 int board_uart_write(void const * buf, int len)
 {
+  (void) buf; (void) len;
   return 0;
 }