add -Wextra for more warnings to example

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

hw/bsp/pca10056/board.mk

@@ -9,7 +9,10 @@ CFLAGS += \
   -DCFG_TUSB_MCU=OPT_MCU_NRF5X
 
 # nrfx issue undef _ARMCC_VERSION usage https://github.com/NordicSemiconductor/nrfx/issues/49
-CFLAGS += -Wno-error=undef 
+CFLAGS += -Wno-error=undef -Wno-error=unused-parameter
+
+# due to tusb_hal_nrf_power_event
+CFLAGS += -Wno-error=cast-function-type
 
 # All source paths should be relative to the top level.
 LD_FILE = hw/mcu/nordic/nrfx/mdk/nrf52840_xxaa.ld

hw/bsp/pca10056/pca10056.c

@@ -96,7 +96,8 @@ void board_init(void)
     nrfx_power_init(&pwr_cfg);
 
     // Register tusb function as USB power handler
-    const nrfx_power_usbevt_config_t config = { .handler = (nrfx_power_usb_event_handler_t) tusb_hal_nrf_power_event };
+    // cause cast-function-type warning
+    const nrfx_power_usbevt_config_t config = { .handler = ((nrfx_power_usb_event_handler_t) tusb_hal_nrf_power_event) };
     nrfx_power_usbevt_init(&config);
 
     nrfx_power_usbevt_enable();