logging: Allow for compilation without log_output

Add option to Kconfig to enable log_output module. It is used
by most of the backends but it is an optional formatter helper
thus it is possible to run logging without it. One example might
be dictionary based logging which does not format log message
to a readable string.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>