nrf51822/log: Parametrize buffer length
radio: Add radio_prepare() API
nrf51822/radio: Add radio_prepare() stub
ll: Call radio_prepare() before use new ch/aa/crc
examples: Call radio_prepare() before use new ch/aa/crc
nrf51822/radio: Implement radio_prepare() API
radio: Remove ch/aa/crcinit params from radio_send/recv() APIs
examples/radio-scannable-broadcaster: Replace radio_reply() with radio_send()
radio: Remove radio_reply() API
radio: Remove len parameter from radio_send() API
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.
nrf51822/radio: Comment ref for radio mode
nrf51822/radio: Set inter frame space to 150 us
examples: Add radio-active-observer example
WIP