logging: frontend: stmesp: Do not append strings when not needed

Currently we can assume that flpr and ppr cores are always owned by
cpuapp. In that case, cpuapp (which processes ETR buffer content) has
access to the memory where logging strings from ppr and flpr are
located. Those strings do not need to be appended to the message
which is written to the STM (in case of standalone configuration).
This approach reduced logging time, e.g. logging a message with
one argument takes 1.8us on cpuapp and 6.3us on cpurad.

This setting applies only to the case when cpuapp is the owner
of those co-oprocessors and shall be updated if that becomes
configurable but currently there are no means to detect core
owner.

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