logging: Cleanup in the internal headers

Added log_internal.h for internal APIs. Move functions out of
log_core.h. Log_core.h shall have only macros and functions related
to logging message creation. Log_core.h is included by log.h thus
number of dependencies in that headers must be limited to minimum
to allow including without risk of circular dependencies.

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