logging: Tweaks to speed up compilation

Logging attempts to do most of the work at compile time instead
of preprocessor by using conditions which can be resolved at
compile time (e.g. if (IS_ENABLED(...))). Apparently, such extensive
use significantly loads the compiler since all paths are compiled
even though cut from final binary. Patch reduces it by replacing
some compile time switch with preprocessor.

Handling of function name prefix has been moved to preprocessor.
Immediate logging v2 has also been moved to preprocessor.

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