log: Replace log functions with inline stub ones when log is disabled
nrf51822/log: Remove duplicated include
nrf51822/log: Change baud rate to 1 Msps
nrf51822/log: Parametrize baud rate
nrf51822/log: Refactor buffer manipulation

Now the buffer can accomodate several messages. Then log_print()
doesn't write directly in the buffer. It first formats the message in
a temporary buffer and, with the final length, copy the formatted
message to the output buffer.

This means that log_print() slower than before, but the atomic
logging functions (in the next commits) will be very fast.
nrf51822/log: Decrease the UART interrupt priority

Now the priority is lower than the RADIO and TIMER0.
log: Add several logging functions
log: Change log_print() API to log_printf()
examples/logging: Update example to use new functions
timer: Rename timer_cb to timer_cb_t
timer: Move cb parameter from timer_create() to timer_start()
nrf51822/timer: Fix timing drift in repeated timers