nrf5: Add default config for MICROPY_PY_BLE_NUS (0)

Disable Bluetooth UART to be used for REPL by default. Can be overridden
in nrf5_sdk_conf.h. It is defined in  mpconfigport.h as it is connected to
mphalport.c, where the config is used to determine whether default print
functions should be using HW UART or Bluetooth UART.

nrf5/mpconfigport.h

@@ -213,6 +213,10 @@
 #define MICROPY_PY_UBLUEPY                       (0)
 #endif
 
+#ifndef MICROPY_PY_BLE_NUS
+#define MICROPY_PY_BLE_NUS                       (0)
+#endif
+
 // type definitions for the specific machine
 
 #define BYTES_PER_WORD (4)