examples/radio-observer: Remove duplicate timer initialization
ll: Modify timing parameters to uint32_t
bci: Modify timing parameters to uint32_t
timer: Change timer_start() API from ms to us

In several places we need to have timers with microsecond resolution.
This patch modifies the timer_start() API to accept us, adds helper
macros TIMER_MILLIS() and TIMER_SECONDS(), and updates all examples.
ll: Enforce advertise interval to be multiple of 0.625 ms
ll: Rename pdu_type to type

This patch removes redundant prefix in the advertising type field.
ll: Add scannable advertising

This patch adds support for scannable advertising, sending SCAN_RSP if
SCAN_REQ PDU is received. SCAN_REQ PDUs can be sent by the remote if the
advertiser is sending ADV_IND or ADV_SCAN_IND.
example: Add setting scan response data

This patch changes the ll-broadcaster example to send scannable
advertising PDU, and reply device appearance in the scan response
PDU.
nrf51822/log: Parametrize buffer length
nrf51822/radio: Clear rxshort after use it
nrf51822/radio: Add RX option to radio_reply()
nrf51822/radio: Comment ref for radio mode
nrf51822/radio: Set time interval between two packets to 150 us
examples: Add radio-active-observer example