nrf51822/radio: Fix buffer overflow bug

The radio input buffer must be able to store a full packet with 39
octets. But the MAXLEN configuration of NRF_RADIO->PCNF1 register
stores the maximum length of PAYLOAD, which is 37 octets, and not
the full packet.
nrf51822/radio: Remove outdated/unnecessary comments
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: Add active parameter to radio driver callbacks
radio: Add radio_set_callbacks() API
examples: Add radio-active-observer example