add -Wextra for more warnings to example

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

hw/bsp/stm32f767nucleo/board.mk

@@ -9,8 +9,8 @@ CFLAGS += \
   -nostdlib -nostartfiles \
   -DCFG_TUSB_MCU=OPT_MCU_STM32F7
 
-# Suppress following warnings/error is required due to STM32F7xx_HAL_Driver.
-CFLAGS += -Wno-error=shadow 
+# mcu driver cause following warnings
+CFLAGS += -Wno-error=shadow
 
 ST_HAL_DRIVER = hw/mcu/st/st_driver/STM32F7xx_HAL_Driver
 ST_CMSIS = hw/mcu/st/st_driver/CMSIS/Device/ST/STM32F7xx

hw/bsp/stm32f767nucleo/stm32f767nucleo.c

@@ -174,15 +174,16 @@ 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;
 }
 
-
 #if CFG_TUSB_OS  == OPT_OS_NONE
 volatile uint32_t system_ticks = 0;
 void SysTick_Handler (void)