logging: Add buffer flushing on entering panic

It may happen that panic occured while logger backend
was formatting output data. In that case output buffer
could get corrupted as logger assumes that processing
happens in one context only (panic is the only exception).

Added log output buffer flushing on entering panic state.

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