samples: modules: chre: Flush log messages after initialization

On targets where POST_KERNEL initialization routines, executed after
the logging thread has started, take significant amount of time,
log messages produced during CHRE initialization may collide with
printk messages output by the sample. Prevent this by flushing all
log messages right after the initialization is performed.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>