nrf51822/radio: Implement radio_prepare() API
radio: Remove ch/aa/crcinit params from radio_send/recv() APIs
examples: Replace radio_reply() by radio_send()
radio: Remove radio_reply() API
radio: Remove len parameter from radio_send() API
radio: Add flags parameter to radio_send() API
nrf51822/radio: Clear flags when stop the radio
radio: Add radio_set_out_buffer() API
radio: Add flags parameter to radio_recv() API
radio: Add active parameter to radio driver callbacks
radio: Add radio_set_callbacks() API
examples: Add radio-active-observer example
nrf51822/radio: Start TASK_TXEN/RXEN as soon as possible
nrf51822/timer: Fix bug in timer_stop()
nrf51822/timer: Refactor CLR_SET_MASKS macro to a inline function
examples/radio-scannable-broadcaster: Add T_IFS timer
examples/radio-active-observer: Add T_IFS timer
timer: Remove unused user data