shell: Allow custom shell log backend implementation

Shell is capable of acting as the logging backend and multiplex
logging messages with shell content. It expects that logging
messages goes through the logging backend. However, there is an
option that logs go only to the custom logging frontend. In
that case this default approach does not work and logging
frontend cannot be easily integrated with the shell (due to
its custom nature). Add Kconfig option SHELL_LOG_BACKEND_OOB
which prevent compilation of the default shell log backend.
When enabled, application can provide frontend specific
implementation to integrate shell with the logging.

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