Append a serial number prefix to the BT device name

[chrisandreae]

To better differentiate multiple devices over Bluetooth, we'd like to use part of the serial number in the the advertised Bluetooth device name.

Implementation questions: it was necessary to set CONFIG_BT_DEVICE_NAME_DYNAMIC and call bt_set_name, or else the name would change back from the initially advertised bt_device_name to the CONFIG_BT_DEVICE_NAME value. Given that the CONFIG_BT_DEVICE_NAME does seem to be being used, and separately, is there a reason that the name is manually inserted in the advertisement data rather than relying on Zephyr's BT_LE_ADV_OPT_USE_NAME and BT_LE_ADV_OPT_FORCE_NAME_IN_AD options?