build: Allow examples to compile unused platform files

This is needed, for example, to use buttons in nRF1822 platform.
build: Add specific build files in .gitignore

Before, only the "build" path was excluded in .gitignore. But if the
user replaces BUILD_PATH, the files built in compilation will not be
ignored. This patch adds the generated common files and removes the
default build directory.
radio: Add documentation about radio events
ll: Add missing const modifier in ll_advertise_start()
ll: Check for errors in ll_advertise_start()
ll: Check for errors in ll_advertise_stop()
ll: Check for errors in ll_init()
ll: Improve callback handling

This patch modifies the code to only one of the callbacks be
responsible to call radio_send().
nrf51822: Add references for the interrupt levels
nrf51822/timer: Add some documentation and references
nrf51822/timer: Better error handling
nrf51822/timer: Increate default number of timers

Some examples need more timers to work properly.
nrf51822/radio: Update reference
nrf51822/radio: Document radio_set_tx_power()
nrf51822/radio: Document radio_init()
nrf51822/radio: Document ch2freq()
nrf51822/radio: Document radio_send()
nrf51822/radio: Document RADIO_IRQHandler()
nrf51822/radio: Transform COMMON_INITIALIZATION() into a function

This patch transforms the COMMON_INITIALIZATION() macro into a
function called common_init() to increate readability, since it
makes more clear that an error may occur.
examples/timing: Modify example to test more APIs

This patch modifies the timeout function of the first timer to call
timer_stop() after 5 seconds and adds a third timer to test the
TIMER_SINGLESHOT type.