radio: Add flags parameter to radio_send() API
radio: Add radio_set_out_buffer() API
radio: Add flags parameter to radio_recv() API
radio: Remove old event documentation
nrf51822/radio: Fix NULL radio driver bug

The radio should operate normally without the driver.
radio: Modify radio callbacks to return boolean values

Radio callbacks now returns false when they want to stop radio
activity. If they return true, the radio operation isn't modified.