logging: enhanced external logsystems

Introduced interface for efficient logging from external logsystems:
Added handling of vaargs and automatic strdup to macros intended
to be used in logging interface function. Fast path to less then 4
arguments to speed up the execution. Made log_count_args external,
if external logsystem cannot count arguments.

Signed-off-by: Tomasz Konieczny <tomasz.konieczny@nordicsemi.no>