logging: Add option LOG_MSG_APPEND_RO_STRING_LOC

Bring back Kconfig option which was previously removed by
9cd5086407 (previously named LOG_MSG_PKG_ALWAYS_ADD_RO_STRING_IDXS).
Renamed to shorten and be more descriptive. It can be useful for
external backends or frontends.

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