How to config BT_DEVICE_NAME variable when application is running

[Chaoyue Ying]

When I developing application with nrf SDK,I can set device name by sd_ble_gap_device_name_set().

But when I turn to NCS,I should set device name such as  CONFIG_BT_DEVICE_NAME="nRF_DM".

However,I want to change name automatically when application running.

And not only string,I want to assign a char array to device name.

What can I do to arrive it?