nrf51822/log: Parametrize buffer length
examples: Rename example to radio-passive-observer
build: Add "examples" rule
radio: Remove old radio events documentation
nrf51822/radio: Set inter frame space to 150 us
nrf51822/radio: Add reference for radio mode
nrf51822/radio: Create define to base radio shorts
nrf51822/radio: Remove all not base shorts when stop the radio

This prevents triggering unwanted shorts right after stop the radio.
nrf51822/radio: Fix NULL radio driver bug

The radio should operate normally without a driver.