ll: Reorganize code in ll_advertise_start()
radio: Add radio_prepare() API
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: 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.
Revert "radio: Modify radio callbacks to return boolean values"

This reverts commit 4d677ed0d416612dc411241673dfec0333471f8f.
radio: Add active parameter to radio driver callbacks
examples: Add radio-active-observer example
radio: Add radio_set_callbacks() API