build: Modify Makefile to allow build configuration

This patch allows the user to configure the build through make command
line. For example:

	$ make CONFIG1=value1 CONFIG2=value2

This defines CONFIG1 and CONFIG2 in all source files with the specified
values.

This is a temporary solution until a better build system is deployed.
nrf51822/log: Add missing newline at ending of file
nrf51822/log: Move code around to enhance legibility
log: Remove unnecessary do-while loop
log: Add CONFIG_LOG_ENABLE configuration

This patch adds the CONFIG_LOG_ENABLE configuration. Its default value
is active. To deactivate LOG messages, build the library with
CONFIG_LOG_ENABLE=0:

	$ make CONFIG_LOG_ENABLE=0