Add USBConsole.h so that default printf can be sent to USBSerial

Author: mjwinney

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_NRF52840_FEATHER/USBConsole.h

@@ -0,0 +1,7 @@
+#include "USBSerial.h"
+
+FileHandle* mbed::mbed_override_console(int)
+{
+    static USBSerial  usbSerialConsole;
+    return &usbSerialConsole;
+}