samples: Bluetooth: hci_uart: Implement NOP Command Complete

Implement the Controller counterpart to CONFIG_BT_WAIT_NOP so that it
issues a NOP Command Complete event after booting up, to signal to the
Host that it is ready to receive HCI traffic.

Fixes #15333

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>