Nordic "nRF52840 Dongle" Support

jsiSetConsoleDevice was only being called once, so it looked like it's something to do with Espruino->Bluetooth rather than something weird happening with the serial or USB ports. Figured it was a buffer not being cleared somewhere so I scanned through likely files until I found a candidate. Changed this in targets/nrf5x/bluetooth.c and it came good:

[#if](https://forum.espruino.com/search/?q=%23if) NRF_SD_BLE_API_VERSION>5
    uint32_t err_code = ble_nus_data_send(&m_nus, nusTxBuf, &nuxTxBufLength, m_peripheral_conn_handle);
    if (err_code == NRF_SUCCESS) nuxTxBufLength=0; //<- added this line
[#elif](https://forum.espruino.com/search/?q=%23elif) NRF_SD_BLE_API_VERSION<5
    uint32_t err_code = ble_nus_string_send(&m_nus, nusTxBuf, nuxTxBufLength);
    if (err_code == NRF_SUCCESS) nuxTxBufLength=0; // everything sent Ok