logging: Use runtime packaging in immediate mode

Always use runtime packaging in immediate mode. Removing attempt to
package on stack if possible. It would be done only to speed up
logging but that is not a requirement in immediate mode where
packaging time is fractional compared to backend processing. Using
runtime packaging also reduces stack usage in the context which calls
log message.

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