build: Add Makefile.common
nrf51822: Add Makefile.platform and flasher
log: Add log interface

This patch adds the interface which every platform should implement to have
logging capabilities.
nrf51822: Implement log module

This patch implements the log interface for Nordic's nRF51822 chip. The
implementation uses the Nordic SDK's Simple UART module with baud rate
equal to 38400.
nrf51822: Add logging example