ll: implement ack/flow control mechanism and centralize outgoing packets preparation
ll: implement a RX timeout timer
nrf51822/random : Fix bug that caused RNG to generate always the same number
nrf51822/radio: add debug option to visualize RX & TX timings on a GPIO pin
ll: Fix bugs in data channel map and index selection
ll: Various debug messages modifications
ll: changed default connection interval to 20ms
examples: add ll-initiator example