logging: Fix support for CONFIG_LOG_MSG_APPEND_RO_STRING_LOC

There is an Kconfig option which results in RO string locations
being appended to the cbprintf package of a log message. Option was
not correctly handled, especially optimized API which was recently
added did not support that. Restoring support.

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